I commute/run errands on a fixed Motobecane Messenger.

Main reasons being:

1. It was cheap.

2. I'm not beating up my road bike or mountain bike, neither of those were cheap.

3. I don't feel any guilt for scratching it up, it's a beater!

4. I like the fixed mainly because I never have to worry about cheap derailleurs needing adjustment, etc.

I think if you're going to have a bike to "Commute" on, get around town, you'll want something cheap and dependable that you're not afraid to scratch up. I avoided track bikes mainly because I wanted a little slacker geometry and I wanted brakes.

In my experience, brakes are definitely key in traffic.