BEWARE: Even if you accept the service deficiencies that you'll read about in the rest of the reviews here (and elsewhere), there's still the very realistic risk of you ending up with fraudulent charges on your credit card.
In my case, a member (or ZipCar employee) lost/misplaced/stole the key to a car that I had days before, and ZipCar pinned it on me, among others, with absolutely no explanation. Calls to their customer service are often hung up on/put into a permanent hold. The explanation/justification they give is that in cases like this, they "investigate" the last few members who had the car to determine who is "most likely" to be responsible. In my case, I returned the car to the home location and ZipCar themselves confirmed that there were others in the car after me, and yet I was still being help responsible.
They never resolved this matter, but continued to charge the monthly fee while my ZipCar account was frozen. Easily disputed this with the credit card company, but the spent hours upon hours on this.
Don't bother; it's not worth it.