While fixed gear bikes are trendy and part of the messenger sub-culture, the reality is that they ride just about every kind of road bike, and even a few mtn bikes set up for road use. Since it's business and time is money, they prefer light fast bikes, but also try to keep it cheap in case of theft or crash.

Here in NYC fixed gear is fine for those who serve mainly lower Manhattan because it's flat, but where hills are a factor, the cool factor of a fixed gear bike gives way to practical realities and you'll see more derailleurs.