For the past two and a half years, my commute has been slowly wearing me out and killing me. So for the past week I've been driving to work, and the difference is profound. When I get to work I'm immediately productive instead of taking an hour or so to get into the swing of things. When I get home at night, even after an 11-hour workday (due to my own recent obsessiveness over my project) I still feel energetic and don't feel like just collapsing on the couch and crying for an hour. It's an amazing difference.
I'll still try to ride my bike on nice days (since it's good exercise and doesn't wear me out like the bus or BART), but holy cow, I am so over mass transit now, at least for my daily commute. (I'll still use it for going downtown to meet up with people, since it's still cheaper and quicker than parking and of course leaves more drinking opportunity and whatever, but work pays for my work parking.)
Basically, San Francisco mass transit sucks and it was pretty much killing me. And I'd rather be alive (and even happy!) than smug.