I rode my Cannondale Mt. Bike to work the other day. I live about 8 miles from work and there are sidewalks the entire way. It took me 35 minutes because sidewalks tend to be slower the street. Like Rick mentioned, I'm not interested in getting hit by a car either. I've ridden road bikes in the past but I got a mountain bike because I could be a little more wreckless with it. The only thing I didn't like about riding to work was having to have my shower stuff and lunch pre-staged. The only thing I didn't like about the actual trip was the afternoon heat, here in Florida, made sweat run into my eyes which tends to make thing a little uncomfortable. Other than that I intend or riding more often because my fat-ass could use some excersize.
$850 new
I got it for $650 as a closeout

http://img126.imageshack.us/img126/9448/cannon1pi1.jpg