Erasmus is a programme by the European Union that, among other things, offers students from European universities grants to study abroad in exchange semesters. You can read more about it on the website. After being part of the programme and having time to think things over, I'm writing this post in order to hopefully help others in deciding whether doing an exchange semester via Erasmus is worth it or not - spoiler alert, I think there are some situations in which it might not be worth it.
First of all, let's make sure we are on the same page and think about what the benefits of being part of Erasmus.
You get to live abroad, on your own. This is good in two ways - one, maybe you never lived alone before, so it is a good occasion to start improving your cooking skills and becoming more responsible; two, you have the opportunity to experience a new city, one in which you might want to relocate for a full time job later on. With going abroad, you also get to meet new people, experience different cultures and improve your insert foreign language here skills. Of course, on top of all, since this is an educational grant, you get to study classes that maybe you would not be able to at your home university.
As you can see, most of the advantages I've listed are not directly related to school. This is not a coincidence. I feel that the educational aspect can be hit and miss - here's why.
You get to go to a new university, with new teachers, new classes and new students - that's a lot of new, which could also stand for might be good but not necessarily. Maybe the classes you end up taking sound great on paper, but when you actually get to class you see that things are nowhere near what you expected. The style that people at the university are used to might not be for you - maybe you do really research-y and theoretical stuff at home, and now you have to do applied, no questions asked, monkey work. And the thing that's the riskiest of them all - most likely you will have to do group projects. To do them, you need to join/form a team. Students at your new university probably already know each other for quite some time and know who to pair up with. You don't - what if you end up with the guy who doesn't care about school at all?
The examples listed above have to do with school itself, but you should also consider aspects like cost of living and bureaucracy. The grant alone most probably will not be enough for you to live abroad - you need to look for other ways of adding to your income (another grant, a part time job, being research assistant at the university, anything). Make sure to research this aspect beforehand. The red tape is a killer as well - I had colleagues who had to go back to their home country during the programme to personally hand in a paper, because the receiving party did not accept fax or e-mail.
As you might already suspect, the school part wasn't that great in my experience. The classes I took sounded really interesting and advanced (i.e. required extensive background and whatnot), and ended up being like tutorials. The hands-on sessions were (literally!) copy-pasting some code from Dropbox and running it locally - there, you just implemented a recommender system, how do you like it? I ended up in a team with a student that was in their 7th (seventh, yes) year, although the bachelor's degree was 3 years and the class was part of the 1st year of the master's programme.
In short, the school aspect was really lacking in my experience, although I went to (supposedly) one of the better universities. However, the experience wasn't all bad - I got to live in Amsterdam for 5 months, and let me tell you, Amsterdam is a great city! Besides, my girlfriend was there as well so that was nice too.
Protip - when you choose the place to have your exchange programme in, don't consider just the university. Make sure to choose a city that presents you with things to see and do, so that you can be sure at least one of the two (school, location) will be pleasing.
Before ending, since you probably are interested in doing something abroad, let me remind you there is one more way to achieve this - internships. They are a great way to live abroad, gain valuable experience and do this without worrying about income (assuming you do a paid internship). I also wrote about one of my internships at Mozilla here.
To sum up, if you have the opportunity of doing internships and are already part of a good university, I suggest you skip Erasmus and go the internships route. To me, this way has less things that can go wrong and the chances of enjoying your time and actually gaining something are higher. Why not both Erasmus and internships, you might ask? It's only a matter of having the time and accepting the fact of being away all the time. If those work with you, go ahead!
Keep in mind, this is only based on my experience. Yours and other's might be different, so make sure to do more research. Good luck!