I have been in Berlin for almost 6 months. Berlin is a truly unique city and incredibly diverse. Unlike other big European cities like Paris and London, Berlin is rather relaxed. There is never a shortage of things to do. Berlin offers events and activities for anybody. In addition, there are so many bars, restaurants and clubs.
I like the fact that Berlin has so many different faces. There is just so much to see. I love sitting close to the Canal in Kreuzberg where you'll find hundreds of people having bbq, drinking wine, reading etc. In some areas of Mitte, you can experience the posh side of Berlin. Prenzlauer Berg is a paradise for families. Friedrichshain has amazing clubs and hidden bars.
The sky in Berlin and the sunsets are simply unique.
There is a growing start-up scene in Berlin which also attracts cool folks (although, no comparison to San Francisco).
During the summer, Berlin is simply amazing with the lakes around Berlin and the Baltic Sea close by (google Darss to see how pretty the beaches are).
Living in Berlin means never getting bored and constantly being surprised by the city. Berlin is open, tolerant, a bit lazy and disorganized and pretty rough during the winter.
There is currently no other big city in Europe that I like more.