Agree with Guennael.
So every office will have their own culture. (WARNING: Stereotypes ahead!)
- Berlin - lots of hipster beards
- Munich - posh, skiing and Oktoberfest are the two most important seasons of the year
- Hamburg - posh, but less in your face about it than Munich.
- Köln - probably the mellowest, go crazy around Carnival. People are quite open and approachable. A lot easier to make friends there than in Hamburg or Munich.
- Düsseldorf - quite similar to Köln, just a bit posher. Carnival is true there as well
- Frankfurt: Why would you go to Frankfurt? ;-)
As you will have guessed by now, there are pointed over-generalisations. However, each office does have their own flavour, which comes from the city it is based in (and obviously the people self-selecting to that office/city). Every office is proud of its city and probably considers it the best / coolest / nicest city in Germany. But it's usually just friendly banter. Except for Cologne and Düsseldorf, they're mortal enemies ;-)