Zipcar is generally a good car-sharing service with absolutely terrible customer service. If you have an alternative, go somewhere else.
I've been a customer since 2011, and have cancelled my account twice (switching to alternative car sharing services) due to horrendous customer service events.
I'm back using Zipcar now due to its convenience factor (cars parked in my building), but Zipcar took 6 weeks, 6 emails, and 5+ phone calls to get me set up. The company is spectacularly inept.
Aside from the occasional flat tire, empty fuel tank or dirty or late car (perils of all car sharing services), the actual act of using their cars is fine.