I am a happy Flexcar member in Atlanta, their newest city (launched last fall). Just used it for an IKEA run this weekend.
BTW, a question for folks in more established car-sharing cities... do you have a sense on roughly how many hours a day a car needs to be in use for it to be considered a successful, viable location? I'm trying to gauge the early performance of the Atlanta service... a number of the cars are doing pretty well, but then there are some that seem to sit unused for most of the day.