Let me explain this to you. If you go out after work you'll find the city completely DEAD.
There's much more going on at the weekends but mostly drunk students or in overly priced business bars.
I'm not saying that there isn't anything to do.
You have a lot of different restaurants with dishes from all around the world, some good bars, a couple of really good art museums but that's basically it.
Like Jorge said it is nothing compared to London.
Last time I came back from a trip from Berlin Frankfurt seemed more like a small town to me.