I haven't actually studied there myself but I know a couple people who did and they were full of praise. I know someone who is doing an economics PhD at the Humboldt Univrsity, and he is really happy there. Good research atmosphere, nice staff and very capable and friendly professors. Of course, this could be different in other departments, but the university has a good reputation in Germany.
Also, Berlin is awesome and much cheaper than other European cities of its size.