I wouldn't call DC a great commuting city- it has lots of good bike paths but is also fairly un-bike-friendly on the roads. But at the same time, the quality of life to be gained by switching from the car to the bike is pretty significant, due to the horrendous traffic in the area.

To wit- the two commutes I've had in the area:

#1
Bike- 25min each way
Car- 10 min morning, 20-30 min evening

#2
Bike- 45-50min each way
Car- 30min morning, 40+min evening

So for a small additional time outlay each day, I gain a solid amount of exercise and don't have to sit in traffic.