1) Availability There's no way you can reserve a ZipCar exactly WHEN you need it. Most ZipCars are spread evenly between locations and in a popular ones you can only reserve something available a few days in advance. This makes the idea of using ZipCar useless because now you need to waste an hour of your time commuting to a remote ZipCar location. Weekends? Forget about it. Unless again you'd like to travel to an inconvenient remote location or book 3 days in advance.
2) Missing/broken car - ruined plans It happened to me twice but this was enough to make me reconsider ZipCar. Once I reserved a car for a day for a very important trip but when I showed up at a parking location my car was missing. ZipCar CSR on the phone refunded my order and offered drive credits but has no idea of what was going on. Needless to say how frustrating the whole experience was. The second time my reserved car was out of order, it just simply wouldn't drive normally. No way I would be happy to exchange this stress and ruined plans for 50$ in driving credits!
3) Dirty Car/low on fuel A couple of times the car I reserved was as dirty inside/outside as it can be after days of abuse on the country roads or in the mud races. And the only thing I could do is to report the issue to ZipCar. I still have to use this dirty car for my business trip.
The other annoying issue has been a car low on fuel. Despite taking a car for a couple of hours the first thing I have to do was to locate a gas station and fuel the car.
4) Overpayment and stress First of all, advertised "starting as low as" prices are non existent or impossible to find. They say you save money using ZipCar. This might be true if you're using ZipCar a couple times a month for a few hours. If you need a car for more than that you're certainly better using a conventional rental company.
Considering that a few rental companies offer "driver pickup and dropoff" service it makes ZipCar highly inconvenient and stressful if you think about all above mentioned issues. I personally found myself extending reservation hours a few times, hurring up like crazy when there was not an option to do it. 5 or more hours using ZipCar equals in price to a 24hrs. using almost any other car rental company.
5) Damage horror stories ZipCar damage waiver is a joke. You can buy an extended waiver which covers only 750$ in damage. Again unlike other conventional rental companies which would happily provide you with a full damage waiver.
Fortunately it didn't happened to me but there's a bunch of horror stories around when ZipCar users were asked to pay fully for the damaged car or had to fight with ZipCar for a long time even if it wasn't their fault.