Zipcar is outstanding. My wife gave it four stars but I'll give it 5. We do not own the car because we live downtown and walk to work. But, once in a while it's convenient to drive around. The zipcars are all over town, parked in all kinds of places that you didn't know existed. Our zipcars are parked right across the street from our condo. They are always there when you have reserved them and clean and ready to go.
No need to pay for gas -- there is a credit card in the car you can use when you need gas. Also, no need to worry about insurance -- it's covered.
Two very cool things: 1. you can use your zip card to reserve cars in many other cities in North America and even London. So, if you travel to NY or Chicago or wherever, you can just book a car online. 2. There are discounts at many stores, hotels, spas, etc. with the card. There is a list of places where the zip card gets you a discount on the web site.
This is a great idea and cannot recommend it more highly.