I was bike commuting for a while, then switched to gym-time (free at work) for about a year to support a co-worker. Having done both, I've dropped far more weight commuting by bicycle (average maybe 5-6 hours a week) than I did spending 5 hours a week in the gym. I (5'10") went from 185 to almost 140 (probably underweight), then I got married and I've rebounded to about 160. But my wife also noted the other day that I have more muscle then I did when we got married.

And riding outside is 97 times more fun than sweating profusely in a warm gym in front of CNN (or your electronic addiction of choice).

Going to the gym might make you healthy, but riding to work can change your life.