I have several route, just for variety, and to add a few miles on a nice day, or shorten it slightly if it's really cold. I go from Falls Church, VA into DC, and usually about half of that is on a bike path. The path is slightly longer than the on-street route, but avoids a ton on stop signs and lights. It's actually probably as dangerous as the streets on a nice afternoon, since some of the roller-bladers, joggers, and nice-day bike riders don't have a clue about other traffic.
Long time bike commuter. I bike commute primarily for exercise and the freedom and independence of not having to deal with a carpool schedule. Parking without being in a carpool is Very expensive. Biking is actually faster door-to-door than taking public transportation.
I've commuted on a road bike, mountain bike, and on a track bike for the past 4 years. The track bike is the best workout, and is a simple machine with almost no adjustments and nothing to go wrong, so it's a great bike for commuting. I do get a few strange looks when people notice the lack of gears and the inability to coast.