This is one of my top reasons for riding to work. I have a tough time figuring out the hierarchy of importance, but I think it goes something like this:
#1 - Time. It takes me 25-30m by bike or 1hr+ by bus.
#2 - Environment. I wouldn't consider myself an environmentalist by any means, but it's nice not to spew CO2 on my commute and equally nice to reduce my financial support of Big Oil.
#3 - Cost. I get PAID to bike to work. Otherwise it'd be $150/mo to park or $850/yr to bus. (This might really be #2 , but I like to think it's #3 .)
#4 - Health. I should prioritize this higher, but it's just an ancillary benefit.
#5 - Ego. I get to feel superior to people who drive to work. (Only kinda kidding!)