I used it with a friend when we were traveling in Oregon for a week. It was pretty awesome.
The issue is that the people renting it out are, well... people. You have less control over how well the cars are serviced etc. Also, you might have to go pick up the car near the owners home, which might not be that close to an airport.
I've used it a few other times as well and I honestly like it. Possibly more so than zipcar.