Haven't heard much on fixies yet.

My first commuter was a mountain bike and I still use it for snow rides. Now, I take my short commute (2.5 miles) on a bianchi nuovo that I've converted to a fixed gear.

The bike is small, light, agile, and fast. I can blaze my way to work in a hurry with very little effort. I've also got a seat post rack, plenty of lighting, and a messenger sack that I carry all my gear in.

I ride my mountain bike on particularly nasty/snowy days, but my fixie does alright in the rain. When I take my mountain bike, I have the advantage of an early morning trail ride (there's a trail near my office), or taking shortcuts through rough terrain.

Most of my commute is just smooth pavement, though, so I really like the speed of the fixed gear. It's nice being able to flow with traffic as opposed to being passed frequently.