I'm a ZipCar member, and very happy with the service. The pricing is aimed at short journeys in London, so exactly what's needed for helping someone move to a new flat, which was the last time I used it.

Otherwise I use it for trips out of London, when the public transport to wherever I'm going is not good, when the flexibility and time saving makes it worth paying more than the train, or when three or more are travelling together, so the shared costs beats the train.