Carsharing is a very useful thing. If you're living in downtown Ithaca and only need a car sometimes, you use Ithaca CarShare. ZipCar was asked to enter the market and declined, so some locals set up a not-for-profit and did a better job.
There are quite a lot of other not-for-profit ZipCar-like organizations in other cities:
http://www.ithacacarshare.org/csh_roaming.html, http://www.carsharing.org/member-organizations/ , etc. (A couple of the ones listed here are for-profit, but most are not-for-profit.)
The fatal problem with ZipCar and most of these others for a non-local is that they have a "home location" for a car, which means it's not too great unless there's one next to your hotel.
So, every time I have needed to rent a car in a city, traditional car rental has been more suitable than any of this. (I can't even use the carshare locally, because I live too far outside of town.)
Car2go is more suitable for "just arrived at a station, need a short trip across town" situations. Car2go is attempting to redress the "must return it where you picked it up, and long rentals are expensive" problem with ZipCar.
However, most times I've rented a car it's becaue I needed to drive 60 miles away from the train station (or airport, before I stopped flying). Hence, traditional car rental.