I joined Zipcar before I had the means to get my own wheels... And what did I do foolishly as soon as I had the means? Yes, I got a car. Sadly this car is painfully expensive now that my starving artist income has returned.
I do still maintain my Zip membership because it's super handy to have access to all sorts of various sized vehicles. Also, if my wife takes the car for a few days to visit family, I'm not SOL if I have a gig. The good part of maintaining a membership, even if you don't have your own car and even if you don't use it... it maintains your insurance history so if you ever do need to become a cage-dwelling 9-5 rage-o-holic, then at least your insurance will start off cheap until you plough into the back of a cop car while texting and drunk on your way to work in the "crush" hour traffic.
I suspect I might return to carlessness with the help of Zipcar in the future... Probably when my current vehicle becomes too expensive to maintain on my budget of $12 per month.