i have just started and i love it. i take BART most of the way, but i bike about 5 miles total and in the shape i am in right now 5 miles is just about right. i haven't had the best of luck so far because i have only biked twice and have already gotten a flat, broken my derailer, and i almost got run over by an old guy who ran a red light.
i got a old Peugeot from the 80's for free and put like $100 freshening it up and it rides nice, i just got to get those brakes working better.