I must say from the few German cities that I have been to Berlin is my favourite one. I went there twice, once in 2008 and once this June.
In 2008 there was a place called "Tacheles" that was still open. That was one of the most amazing places I have ever been to. Unfortunately, now it has closed.
The partying scene is absurdly wild. There are still clubs in abandoned warehouses, "impromptu" parties in empty swimming pools, under bridges, in squats ... Of course I know that most of this is rather fake and commercialized nowadays, but it still gives you a small taste of what it must have been like in the early 1990's. In general it is much too hip for me though. Everyone is so incredibly hip that as a non-Berliner you can't help but feel way too lame in comparison.
The historic sites are unique. Personally I recommend the Bernauer Strasse, where the wall is still almost untouched. In June I went to the Treptower Park, where there is a HUGE Sovjet monument, neatly taken care of every day. That is very impressive to see.
I would love to go see the Teufelsberg, where there is a huge abandoned US-espionage complex from the cold war. There have been organised tours for 2 years now but people have been going there illegaly for decades so it must look pretty cool. You can also visit the Olympic Village of 1936 (?), which also seems quite impressive, but I believe it is a bit outside of the city.
So in general I would say it is a fun city in the sense that there are many unconventional things for visitors to do, that other cities might not offer. But I would not want to live there. In general, I am more of a francophile though so that doesn't have anything to do with Berlin in specific.
I think German Band Kraftklub got it spot on: "Even though other cities suck, I don't want to go to Berlin"