Typically have been satisfied with Zipcar, but recently had an experience that really peeved me. They cancelled my rental for a Monday (SUV) which was around St. George Station - no explanation given. They had 5 available models when I rented. I get an email a few days before my rent day saying they've switched me to a sedan because of an unforeseen reason. Normally this wouldn't piss me off, but when I cancelled the sedan and went online to rent an SUV, they once again had 5 SUV's, but the St. George one had been moved to Spadina it seemed. So I went on and rented it. Just annoyed that they wouldn't offer the same vehicle but change location - what if I couldn't get an SUV, which is the vehicle type I needed.
Anyway, overall service is OK. Have started using enterprise a lot more as they have pretty good deals frequently. I find Zipcar works best on weekends when daily rentals at enterprise don't make sense.