I just came across this post over at Get Rich Slowly, after I got linked there by a post on buying used cars at The Art of Manliness. Its a good idea, and one I think my girlfriend and I will have to start with when it comes time for us to buy our first car. Right now we are carless, we live in the downtown area of our city and simply walk or take the bus everywhere. When we need to we do borrow a car from her parents, and we have considered a Car Share membership in the past.
I like the idea of car sharing, I think it is great for the environment and perfect for people who don't want to maintain a car and don't want to drive everywhere. That said at the end of the day your money is essentially gone. Buying a good, cheap used car (and still not driving everywhere) is probably the better option.