Can't speak for the rest of the world, where things may be different (owing to different laws, culture, fuel costs, etc.), but here in the UK I've found it massively convenient. I was a member of StreetCar for a while before they were taken over by Zipcar, and the whole thing has been really good.

In my case, I live in Oxford, which is a pretty car-hostile city: little on-street parking, priority given to buses and bikes, expensive car parks, etc. I don't own a car, and - for the amount that I drive - it would cost me more to own and run (maintenance, duty, insurance, etc.) that it does to Zip, so that really works out well.

I Zip about one evening a week, termtime, to get to college in a town about 20 miles away. Renting a Zipcar gets you 40 miles travel included, per day, so that kind of journey length is just about the "sweet spot". I sometimes borrow one on a weekend to go do some paramotoring. It's convenient to be able to pick up/drop off a car only when I need one, and not think about one the rest of the time.

I've had only a handful of issues, ever. One came when the car I'd booked was returned late by the previous customer. It was the previous customer's first time renting a Zipcar, and (apparently; so Zipcar tell me their statistics show, new users are most-likely to miss return deadlines) he got it back about half an hour late, which threw my schedule off too. Zipcar worked hard to try to find my another conveniently-nearby car in their fleet to switch me to, and refunded me the difference in lost time, but it was still a little inconvenient. That was a year and a half ago, and since their fleets have been increased in size around here, it's not been an issue again.

Their customer service is very good, and their web interface is friendly. You can pay an extra annual premium to reduce your excess in case of an accident, but I found that it's cheaper to get third-party excess reduction insurance (by about half), so I'd recommend that instead. We also use Zipcar For Business at a non-profit company that I run, which works out pretty well - it's often easier than calculating mileage for our volunteers using their own cars, and it means that we're quickly able to leverage a whole fleet of vehicles if we need one: at a conference we ran last year, it was great to be able to just grab as many cars as we needed, for the day, transport equipment around, and dump them back afterwards.

I wish the nearest Zipcar to me wasn't parked three quarters of a mile away, though! I usually cycle out to collect it, but I'd far rather there was one on my street. Maybe someday: they seem to keep adding more!