Why did I apply to go to Leiden?
There were several reasons. First of all, it was an adventure. I am not from Stockholm and I don't plan to live the rest of my life in Sweden. The possibility of spending an entire semester a little bit more south - although it's not exactly the tropics - was very appealing. I knew practically nothing about the netherlands except what everybody knows, which is mainly Amsterdam, canals and legalized drugs, and so I was curious to know what dutch people are like and what day-to-day life looks like.
I've previously lived abroad in the french Alps and the USA and they were great experiences. Meeting new people and just being somewhere new is something I find very exciting and you always learn something new on these excursions. I can see why it's considered a merit on your CV and of course it was also considered a merit for biomedical sciences students at KI, which was also a reason for going.
Last but not least, I was not so excited about Leiden (although I was still applying since it was my chance to escape Stockholm) until I listened to the girl who just got back from her exchange there. That's when I really realized that although an unknown city in the netherlands might not seem really exciting at first thought, it's still in the middle of europe. By train or flight you can easily go to Germany, France, England, Berlin, Spain and Denmark very cheaply, and you're a 35 minute train ride from Amsterdam. With a little bit of saved money, the erasmus scholarship and your CSN-loans you have a unique opportunity to see a lot of europe and have a lot of fun!      

What to think about before leaving
Doing an exchange within the european union doesn't really require much from you. You don't need any special visas or so. Just make sure your passport is up to date and you have that blue european health insurance-card which you can get for free from I think skatteverket. Ask the coordinator, she can help you order it immediately.
You will also need a maestro card, no matter what your bank tells you. Your VISA will mostly not work.

Information beforehand
This was an incredibly annoying process. I had told the coordinator I had a room that the students from Leiden could stay in, if I could stay in one of their rooms. I gave this information in may, when we got our nominations. Then I didn't hear a thing. After my application to Leiden university was approved, mails started hailing in every week, from different organisations in Leiden, with papers that needed to be printed, signed and scanned. I was in America due to a scholarship and was doing a research project alongside with excercises that came with the scholarship, so I was already quite busy. End of july, I still hadn't heard anything about swapping of apartments and I had to get in line for Leiden housing which demanded 500 euro deposit in advance (luckily I had this, but I know plenty of people who don't have that kind of buffer) and also sent more papers to be printed, signed and scanned.
The info I got I saved and thought to look into at the end of summer, once going to Leiden, but at that point it was so many different papers saying a thousand things and explaining nothing, I had no clue what was relevant and what it all was for.  
I found out just before leaving for Leiden, that the dutch students had never been offered, by their coordinator, any of the rooms that we swedish students wanted to offer them, and their rooms had randomly been offered to other travel-friends from my class, who had not had any rooms to swap and had not asked about it. 
If you find yourself in my situation, look at the facebook-page Leiden Housing. That's private persons looking for room mates or leasing their apartments for a limited amount of time. Then you don't have to pay the ridiculously exaggerated prises that the "real" Leiden housing makes you pay, and you will most likely end up with a much nicer arrangement.

Arrival
I, just as the rest of the classmates leaving for Leiden, arrived a couple of days before our introduction days (which were one week before the courses started). Lucky for us, it was beautiful days in the middle of a heat wave and it was really nice to have some unscheduled days to explore the town and it's surroundings a little.  
Had I not been in America, I would have definitely done the two weeks dutch-language course that starts two weeks before the introduction week. You get extra Erasmus for it anyway, and it would have been good to learn a bit more about the dutch schooling system before starting the actual exchange. It's also a good opportunity to meet some new people and the Netherlands is typically a bit warmer than Sweden and has quite stable weather in august. (Leiden is situated very close to a fantastic beach by the way, an unexpected and happy surprise.)
There was a "fadder"-organisation of some sort which most of us signed up for, but for some reason I was the only one who actually got one. But I brought some classmates along to the fadder meeting, and it was a fun evening where we learned a bit about the town and then went to the pub called Einstein, which is packed with exchange students on wednesdays and is more or less empty every other day.
The introduction-course was confusing, but good, and a lot of fun.

Economy
Well, housing in the netherlands is typically a little bit more expensive than in Sweden. For me however, who ended up in the worst living arrangement in all of Leiden, at least had it quite cheap. My first thought upon arriving to my place of living was "I can't live here!!!" but it turns out I could.
The deposit of 500euros beforehand was a bit juicy, but I thought of it as saved money instead. 500 euros that would appear in january isn't too bad. I don't remember if I payed anything else beforehand, of course the plane ticket, but I got a cheap one with Norwegian air and so that was about 60 euros with luggage and all.
Of course the price of alcohol is incredibly cheap compared to Sweden (and especially Stockholm). I don't think I ever payed more than 3.50 euros (and that's considered expensive) for a glass of wine and the beer-drinkers had it even more cheap. Also food (supermarkets and restaurants) is a lot cheaper than Stockholm as well.
The trains are quite expensive (not more expensive than SJ though), so for us ending up going to Amsterdam every weekend, it would've probably been cheaper to get that dutch bank-account and then get the 40% discount card that you can buy at the train station.
You will probably need to get a bike as well. This is something you can get cheapest at the introduction days or from Leiden housing (the facebook page). When you get a bike, invest in a good lock, one that has something in front of the  key-hole and/or a spray that you can spray into the keyhole for when the rain or frost jams your lock so you can't get the key into it or turn the key around. (Trust me, this will happen, and it is not fun at 7 a.m. no matter if it's a weekend or weekday). Do not try to force the key to twist if this happens, it  will break inside the lock.
If you want to keep costs down, it is very possible, however I figured that I was now in a pretty much once-in-a-lifetime situation, and so I didn't worry too much about spending money. It's what they're made for.

Housing
I  was not happy about my housing. At all. First of all, everyone else of my class mates were living right in the centre of town or very close to campus. I was not close either to them or to anything else. The nearest supermarket was more than a kilometer away. The campus about two kilometers. And every part of town worth going to about three to four kilometers. Needless to say, a functional bike was absolutely necessary. Dragging my luggage those distances on the first and last days, was also no fun. My area was named Rijnfront, Oostgeest, but was more commonly known by it's inhabitants as "the swamp", because it was built for asylum-seekers as temporary stay, on a semi-drained swamp. You know those pictures you see of old dutch slanted buildings with small winding stairs and views of cobble stone and canals? That was not what my housing looked like. My housing was a collection of barracks, where the inside looked like those awful "kollo" or camps, where schools force high-school students for a cheap and "fun" trip with their classmates. Luckily for me, a very sweet italian girl and a crazy bulgarian girl which I lived with, had arrived a couple of days earlier and cleaned out most of the spiders, old food left in the house and hairs out of the drain. (oh right, I just remembered that I forgot to mention that we payed a 140euro cleaning fee each, on top of the 500 in deposit. Money that was never returned although there had obviously been no cleaning before we came). Since we were living in a swamp, there was for some reason mosquitos all year around as well. And plenty of them. Even though my door and window were ALWAYS closed, four or five of them found their way into my room every night, keeping me awake every other hour by biting my face, buzzing in my ear or simply by the fact that once per night I couldn't stand it and turned on all the lights trying to hunt them. It was no use, they were obviously trained ninja-mosquitos, never visible to the naked eye. After investing in a mosquito-net over my bed in october, I could finally get full nights of sleep, but they still found their way in sometimes. We had a facebook-group called "swamp-dwellers unite!" where people would post the most absurd pictures of dead mosquitos and plenty of mosquito jokes. But the page was also the best thing about living in "the swamp". There I got to know a lot of fun people in our little community, as there where dinner invitations, parties, quiz nights, movie nights and pub crawls posted on this page, as well as things for sale and any kind of help you could need, if it was borrowing sugar or repairing your bike or taking language classes in any language. (it was mainly exchange students living in the swamp, but some "dutchies" as well).
Most housing in Leiden has little communities, but it's mainly in the swamp that it is a big thing. Probably because we were so secluded from the city.
Another nice thing with living in the swamp was that there were very nice areas to go running and the Leiden university gym was very close by, which was to my big surprise actually a very nice gym with fun classes and a big variety of machines. All university tests are held there as well (yes, it is very strange to write a test  in a gym-hall). If you are interested in losing weight, the swamp is also the place for you! With the kitchen that was not fit for cooking and all the cycling back and forth to town, campus and the supermarket, on average the people living there lost about five kilos. I who also went running and dancing in the weekends, lost about twelve and was accused of having become anorectic when I came home for christmas.
Don't worry though, you will not end up in the swamp, because it was demolished in february, as the government had deemed it "not fit to live in".  

The studies
I wish I would've had ANY clue of what I was getting into concerning the studies. Maybe if I would've read the travel reports beforehand... First of all, at KI we have long and many lectures, explaining everything we need to know to pass the exams. This is not the case in Leiden.
I will give you this advice - find out what books you will have for the semester and start reading over the summer.
The Leiden courses are brilliantly laid out. You have a so called "blokboek", telling you exactly what you are supposed to know, and you have questions to every chapter where you are supposed to reflect over what you have read. There are so called "work groups" where you gather to discuss the answer to the questions and you have microscopy where you look at histology slides of the tissues you are learning about. Sometimes they also use little computer programs with info and interactive questions and figures. (By the way, you need to bring your own laptop. If you don't have one, you'll need to buy one, because you can't pass a semester in Leiden without one and they won't provide you with one).
But if you have never worked the way they do, you will get a shock.
One day after the introduction lecture, we had the first work group. To answer the FIVE pages of questions you had no help from the introduction lecture, and you were supposed to read 50 pages in the course book. The day after that one, there was another work group with another three pages of questions to another 50 pages in the course book. And so it continued week after week.
Another difficult thing was using blackboard, their equivalent to PINGPONG. Even with having settings at english, most of the page was not translated and so you had to cruise your way through it in dutch. Not the easiest task I can assure you, although dutch especially in writing isn't very far away from swedish. Your courses were not automatically on your blackboard page, and although they explained a bit of how to find and enroll into your classes on the introduction days (for gods sake take notes!!!!!!! It will save you a lot of trouble), it wasn't very easy.
Check blackboard every day and read everything that comes up about your courses, that was my first mistake not to do. There's another page called USIS where you have a different login and password and you have to enroll in your courses. I don't remember what that was for and I hardly used it, but it was important for something.
The occasional lectures you could in principle skip, they won't give you any help for the exam and are only a simple refresher of what you are already supposed to have read in your books. It is quite cosy to listen to for an hour with a cup of coffee and a stroopwaffel though, and good just to show your face and that you're interested, meet your friends and get up in the morning.
For the first course, most  of our swedish exchange group studied together in the student center called Plexus. It's a nice place to sit (although a bit over air-conditioned, which was unfortunately the case with all study areas, also the hospital), and it has computer rooms, free wifi, a small gym, cheap coffee and a common area where we would discuss the work group questions. We went to every lecture, every work group and read most of the book (the material was very overwhelming though). Still most of us failed the first exam and the ones who passed did it just barely.
Their exams and system is very different from what we are used to at KI. I can't really prepare you for this. My biggest mistake was not working from the blokboek. I worked from the lectures and the work groups. But the blokboek actually does say that you need other details than that. Don't be fooled by the fact that the exam is an "open book"-exam, meaning you can bring the book to the exam. You do not have time to look things up. Memorize the details. Know what the blokboek is telling you to know. Read all the extra articles they post on blackboard. Do not miss any work groups, and if you do, make sure you find out exactly what was said. Write down everything that is said in the work groups.
Don't believe that a 40 hour study-week is remotely close to what's expected of you.
I will say this: the quality of the education was really good but what was frustrating is how much they treat you like an ignorant child. The student-professor relationship is very condescending. Don't let it get to you. Talk with your classmates about questions, that's what the dutch do on their biomedicine facebook page.
The physiology-course is the most difficult one. The exams consists of "multiple choice"-questions. They are a bit like "högskoleprovet", where you really have to focus to understand what the question is, read very carefully and the answers are similar to eachother. On top of that, they will also subtract a certain amount of your correct answers with a so called "guess-score", which will lower your grade, because there is the slight possibility that you just guessed correctly on some questions. Since I am not a lucky guesser, that guess-score is why I failed physiology. Two points can be the difference between a high grade, and failing. I can't give you any tips, just know all the material and good luck. Oh, and do all the polls they post on blackboard. Know them by heart. Several of those questions will appear on the exam.      

Dutch and their language
Of course our lectures and courses were held in english. It is however not our native language and not theirs either, which makes for some hardship when it comes to learning and teaching and also the exams. Sometimes you're just not sure what they mean.
The culture in Leiden I would say, is not very representative for the dutch culture all in all. It is a special student culture from what I've heard and come to understand from being in other places in the netherlands. As an exchange student, you are not likely to get to know a lot of the dutch students. They have sororities and fraternities, like in America, but with even sicker traditions. One of the girls in our class described some of the initiation rites she had to go through to join her sorority (which is apparently synonym with having a great job for the rest of your life, as old sorority members in power positions hold eachothers backs), and what she described was human torture. I was amazed that this could possibly be not only legal but also defended by dutch government. Apparently there was a big discussion when somebody died during these initiations a couple of years ago, but the tradition lives on. To these  sororities and fraternities we were of course not invited, nor were we interested to be. There was however another organisation for the whole school which held parties and banquets. We went to the first party and felt a million years old. If you are 20 when going to Leiden, you will most likely be the oldest at the party, since almost all dutch students start university right when they finish "high school" at eighteen. Most of them are also living on their own for the first time. Well, if you've been to a club full of eighteen-year-olds in Sweden, you can imagine what I'm getting at.
So most likely you'll be hanging out with the people you came with, and other exchange students. A lot of the exchange students are really excited that they can buy and smoke marijuana legally. You will come across it. Most of the dutch don't smoke it and look down on it, so you won't see it at the university. Drug culture is however very strong in the netherlands, especially in club life. Don't worry, you will not notice them unless somebody points them out to you and they won't bother you. They want it for themselves and are not interested in getting you into drugs, and so they won't offer you drugs and they won't drug you. Even if they do offer you drugs, they will only shrug their shoulders and move on if you say no. But if you are the preachy type, you are going to the wrong country.
Something else that could shock you, is their Sinterclaas in the beginning of december. It is some sort of version of christmas, but with dutch people dressing up as elves with black faces and a big white pope-santa. A big debate has arisen whether this is racist or not (the dutch are very defensive about this holiday and claims it is not the least bit racist, their faces are covered in soot!), so it just might be that the elves with black faces will no longer be around this year. Except for this somewhat disturbing feature, it is a quite sweet tradition, but as a swede you will probably miss glögg and lussekatter (if you like that sort of thing), since they don't celebrate christmas properly. You will probably find yourself at IKEA at this point, buying glögg and pepparkakor, and other things you are surprisingly missing. 
Other than this, dutch and swedish culture is not very different from eachother. The dutch culture is a bit more vivid, especially compared to Stockholm, but when it comes to way of thinking we are very much alike.

Learning dutch
There was a dutch course offered by the ISN once we had started school. It however cost 500euros, so at least for me, that was not an option. You understand some words here and there after a while, but the best thing you could do is get a friend who is dutch and ask them to teach you a bit. If you buy a childrens book and google translate and have someone to tell you how to pronounce the words, you're already learning the basics.
You don't really have to learn it, most dutch under 50 years of age speak english, and if they don't you can usually use body language to understand eachother. But they are very impressed and friendlier to you if you learn a bit and it is quite a fun language which is not too hard to learn with all it's similarities to swedish, german, danish and english.

Spare time
Well, if you're doing everything right at Leiden university, you probably won't have a whole lot of spare time.
But as I've previously mentioned, there was a gym-hall close to where I lived, which belonged to the university. Unfortunately for us, it was under renovation most of the five months that we spent in Leiden, so it offered a limited amount of classes and machines. We did however go to what was offered  and we had a lot of fun. For you going there now, they offer so many nice things that we really wanted to try! Not only do they have several different type of dance classes (on top of the things you'll find in every gym such as yoga, core training and aerobics), but they even offer pole fitness, tennis, volleyball, football, running club etc. The facilities are also newly renovated and the machines in the gym are brand new. I would highly recommend getting a gym-card there if you are the type of person who likes to exercise.
Otherwise, the ISN (international student network) is the organisation which organizes social activities for exchange students. If you go to the introduction days you will 100% come in contact with them, otherwise they have office next to Plexus (the student center). They organize the pub/club Einstein every wednesday night, which is mainly for exchange students and they also organize other things every now and then, like a cheap weekend-trip to Antwerp (Belgium). They also lend their office space for small organizations within the ISN such as theatre club, music club, cooking club, art club, whatever-club.
Getting in contact with other exchange students in Leiden is the easiest thing in the world if you go to any of these activities. Your fellow dutch classmates is somewhat more difficult. Or I don't know, maybe it's different from year to year. We organized some small events to try to get to know eachother a bit better. We had one "after exam" lunch, one movie night and one bowling night. Not many dutch students showed and the atmosphere was mostly awkward. We talked with eachother, but we were very un-alike, maybe also because of the age difference.
The first month or so, we absolutely loved Leiden with all it's little cobble stone streets, canals, cheap pubs and restaurants. Every wednesday and saturday there's a market in the center of town (the saturday is the best one), which especially when it gets a little colder and closer to christmas, is very cosy to walk through. You can buy fresh, cheap groceries and cheap other products like things for your hair and make-up etc. and do not miss buying a freshly made stroopwaffle!! On mondays the first month, everyone goes to a pub that turns into a club, which I for the life of me can't remember the name of, but it has "two for one" happy hour, with every beer or glass of wine you buy, you get an extra one. Later on I would go to quiz-nights on monday evenings instead, held in a little english pub close to Plexus.
Every wednesday there's Einstein for the exchange students, which also turns into a club after twelve o clock (be aware that after one o clock you are not allowed in anywhere anymore). In the weekends, the town actually quiets down as most dutch students leave to go home over the weekends.
It is great to never be more than a bike-ride away from anything and you'll find several things in Leiden that you'll grow to love. But after about a month, so around half of october, Leiden really started to feel too small. After a few weekends of just hanging out at eachothers houses, and weekdays where you'd already tired of Einstein and that monday-place because it was always the same thing and the same people, and besides we didn't have time for it either, prioritizing studies and sleep, we started to go to Amsterdam in the weekends. It is a 35-40 minutes train ride, and there you'll find nice shopping and a HUGE variety of restaurants, pubs and clubs. I'll leave it at that, the rest is for you to discover.

How has my semester in Leiden effected me?
Well, it was somewhat of a rollercoaster. I don't regret going, the netherlands is a fascinating country. And I met my current boyfriend that semester, in one of the clubs in Amsterdam. I would never want to go back to Leiden. The professors were arrogant and their attitudes old-fashioned (with few exceptions). Many of the students were elitist and their student-life did not attract me at all. 
As disgusting and ridiculous as my housing was, I do think of it with fondness though. I can clearly remember my little running-route and how beautiful it was in the autumn. Taking the bike to the seaside and coming home to my room mates which could both drive me crazy and make me laugh and whom I had very interesting discussions with.  
We had cards to get into the study rooms at the hospital at any time, and sometimes before big tests, all of us swedish students would sit there from afternoon until the middle of the night/morning, when it would be completely empty. We were a very tight group, and because of the short distances, it was easy to just decide to have dinner together at somebodies house or go for a beer, and we had a lot of fun together both in Leiden and in Amsterdam.
The study methods in Leiden really were good, but if you're not used to them, they're a huge challenge. And the exams did not really reflect the level of the course-material. In a way I wish I could do that whole semester all over again, knowing what I know now.
For me, the Leiden semester unfortunately had a negative effect on my studies. The make up-exams made it impossible for me to give 100% on the present courses. I lost my drive to work harder because it was all too much. Now that I have finished my make up exams however, I think the coming semester will be a good one. I feel "back on track", but it's taken a full year to reach this point.
It's been an incredible experience though, I made a lot of new friends, learned a lot, and at this point it is quite plausible that I will move to the netherlands once my studies in Stockholm are finished. I think this is one of those things I'll be nostalgic about as long as my memory is intact, and I'm very glad that I went.