I've been a member for six months now, using it once or twice a month.
The service is great and convenient. I usually book a week in advance and haven't had any issues getting the car I want. The website and phone app are very easy to use. Also whenever I had to call customer service, the representative is helpful and friendly. However, Zipcar is expensive and generally, you can get a better deal at Discount or Hertz, if that is an option. I'm using a discount given by Seneca College, so that helps a bit. There are also no locations in York Region, which is a real bummer for me because I would need to take a 30 minute bus to the closest Zipcar.
I would recommend this if you Zipcar is your only option and you live near a Zipcar location.