I used the Zipcar for the 3rd time this morning. The second time was a week ago. We had a problem, the zipcard could not unlock the zipcar (Element Estelle) (all cars are alliteratively named for their models.
Fortunately when I called, they were able to unlock the adjacent car for me (Matrix McKenna), which I then took with only a few minute delay. Today I signed up for the second car (the Matrix), figuring it worked last time and the first did not. That was a wise choice, as someone else had reserved the other car this morning and had exactly the same problem I did previously, his card could not unlock Element Estelle. I realize Zipcar will do something for you (I am not clear what they will do), but this is a bad sign in general. Of my four interactions with zipcars, (three of my own and one observed for the other poor soul), two did not work correctly.
Just a comment about using the zipcars: I think that perhaps the U could use a few more at each location.
Quite often my need for a car is fairly spur-of-the-moment (like, I wake up with my back out of whack and need to go see the chiropractor). The zipcars, based on my experience, are NOT really available unless you have a couple of weeks' lead time available to find one. My first zipcar attempt was very successful because it was for a physical: I was able to schedule it "out" a couple of weeks, then find a car.
Since then, though, I've wished I could find a car quickly (within a day or two) three times, and there is NEVER one free.
I'm seriously debating whether to continue my membership. I love the idea, but it isn't working out the way I'd hoped.
Just my two-cents'-worth.