I used to use it out here in Boston, mostly for getting to and from hockey games. As far as I can tell, they didn't bump up the rates at all if you had multiple drivers on the same account, so if you know of anyone who might also be interested, it's a nice way to lower the costs a bit, dividing up that monthly fee.
My former boss also used zipcar for a while up until his kids hit the driving age and he swore by it.
edit: The car selection was kind of crappy though. I guess you can live with driving the most ridiculous looking car if you don't actually own it.