I would definitely return to Berlin, it's a brilliant city with so much going on and a great atmosphere. It's a laid back city, packed full of activity. Finding somewhere to stay was difficult though, as at the beginning of September there are so many students looking for housing. It took me 3 weeks to find a room to rent. I'd recommend going out just before September or trying to organise something via email. I would definitely recommend Berlin; it's very cheap and good for students. There are quite a lot of English speakers there; you have to be quite disciplined to speak German, so make the effort to speak it was as many people as possible.
My year abroad was a fantastic and more-than-worthwhile experience; there is just so much on offer in Berlin and always something new to do. I met some great people there thanks to a really good language assistantship network - I'd really recommend it as you make friends really quickly. Berlin's also quite a friendly city with loads of places to go socialise. Definitely tell others to go!
Berlin is teeming with an amazing range of cheap places to go out and is a very young, lively, interesting city. There's loads to do, with some great bars and clubs to explore. My advice would be to arrive early, do a bit of research before you go especially on accommodation and bureaucracy, and try your best to find a free language course while you’re there. That way, as you learn new vocab and grammar you can compile a list of questions and then have the answers explained properly by a native speaker.