Zipcar is a great service that comes in handy when you need wheels on short notice. If you need a car for longer than a couple hours and have time to pre-plan, I find it more economical to go to a regular car rental agency and get a car for 24 hours...this all depends on your needs of course.
I have been a member for over 3 years now and will continue to keep up my membership because Zipcar has been a lifesaver at times. My only complaint is make sure you don't forget anything in the car - they will make you rent the car for the 1hr minimum to unlock the doors to retrieve your item.