One other thing: although the car hire itself is inexpensive, parking in SF certainly is not (over $40/day at most good hotels), finding other parking is not easy and I find a car is unnecessary in town anyway. You're better off walking - this is one of the world's great walking cities - and using cabs otherwise. Get the car only when you want to leave the city.

BART into town is quick and convenient too: c. 30 mins and $4.95, respectively, to Powell St.