I was with ZipCar for seven years from when they first started out in Toronto and at that time they were great and that's largely because there was only two car sharing services and they were the bigger one. The cars were good, the prices were good you could get a car that was midsize for $6/hr and always great. Now midsize cars cost $14 an hour and the customer service is crap, the locations are only downtown and did I say the customer service is crap because it's really really bad. Not only is the customer service bad but they also charge about twice what any other car sharing service does in Toronto and they don't care if you object to this because when you call their customer service line you're calling the US, their Toronto office employs about 3 people.
They recently renewed my yearly subscription and changed my plan to one that was $65 more than it ever was before without notifying me ahead of time. I called them up and they said they would send it right off to billing to have it refunded because my plan had been changed without my permission. Two days later I hadn't gotten a refund or heard from them so I called them again and was told my account was in review and that I would be contacted that or the next day. Again nothing happened so I called them on the 5th day after the first call, got the same bs that it was in review and that I would hear from them the next day or the day after so I cancelled my account and got a full refund.
My advice, try one of the other care sharing services because Zipcar has gotten complacent and there are better and cheaper options out there.