I arrived in Berlin in September 2012 without accomodation. While I searched for a flat I stayed at a hostel for a day or two, and then after that with the "buddy" TU Berlin had paired me with. He was very friendly and I recommend people to apply for one if they are given the option. I found a flat within the first week using the German website www.wg-gesucht.de. This was very straight forward even though I was being slightly picky - I wanted to make sure I was living in a German speaking flat, and I wanted to live with more than just 1 or 2 other people (in hindsight this might have been a mistake). After sending off about 20-30 emails, I got a response, went round to meet them and then moved in the next day to a flat of 6 (with 5 native German speakers). Although this was undoubtedly a good move for my German, a flat of 6 was a bit too big to become close friends with any of them, as everyone usually disappeared to their rooms after coming back from University.
The beginning of University was a bit daunting, Erasmus students weren't really offered any help (that I was aware of) and it was all a bit confusing and I generally had to ask other students in order to have any idea what I was supposed to be doing. Once I'd chosen my modules and everything and had settled in a bit, it was all really straight forward - studying maths in German is surprisingly easy because the set of vocab you have to learn is very limited and a lot of the words are the same in English and German.
While a lot of Erasmus students took advantage of all the Erasmus parties and trips and events and very quickly met lots of other Erasmus students who would be their friends for the rest of the year, I never attended any of these things and tried instead to make friends with Germans (for sake of improving my German). This was quite difficult as my German wasn't great and it's quite hard to convey any kind of a personality in a language you can't speak properly. Because of this it took a while before I connected properly with anyone, and the first few months were generally quite hard for this reason , but as soon as my German improved a bit and I started to make good friends. By January or February I had a great group of really close friends.
In regards to Berlin, I can't recommend it enough. Summer there was probably the best few months of my life. My biggest hobby is DJing and making electronic music, and Berlin is almost certainly the best place in the world to do this. Once exams were out of the way, a standard day would be something like - meeting friends in one of the many parks, maybe attending one of the vast amounts of open-air concerts/festivals/exhibitions which were pretty much always free, heading round to someone's house to cook dinner together, maybe even a VoKu (http://en.wikipedia.org/wiki/Volxkuche), then heading to one of the many bars/clubs where the DJing standard was always very high and the atmosphere incredible. On weekends the clubs are often open all weekend, and during the summer they usually have a stage outside, which means most weekends we got a good night's sleep on Saturday and then met up on Sunday morning, had breakfast together then headed out to either a club or one of the many "open airs" (i.e. free parties out in the open somewhere) where I experienced some of the most best times in my life so far. On top of all of that, Berlin is very cheap and has pretty much anything I can think of that a big city should have (good infrastructure etc).
Overall, my experience with Erasmus was that it was certainly difficult at some points, but I never for a second regretted my decision to do it, and in the end I had probably the best time of my entire life (and much to my parents dismay, I intend to move back to Berlin and start job hunting as soon as I graduate in June!)