For most times you want to use it (weekend), everyone else does too, so there's never a way to "spontaneously" use a ZipCar instead of something else (you have to reserve days in advance), and it's way more expensive than you'd been led to believe.
But secondly, I discovered hourly-based rentals just don't work. I was the idiot who returned my 4-hour rental an hour late, simply because of factors outside of my control (insanely long lines where I was, too much traffic coming back, but since I don't usually drive or go there, how was I supposed to know?). I felt horrible for the next customer who didn't have a car, had to pay a fine, and realized that life and traffic are just too unpredictable for reserved hourly rentals to make any sense at all.
If ZipCar had a huge fleet where everyone who wanted one in Manhattan on the weekend could get one, that would be great... but then it would be sitting unused during the week, and be way too expensive.