Joe and I have just joined this car club and tried it for the first time yesterday for a trip to visit an elderly relative (this particular trip is very tricky by train).
Booking process was very easy, more straightforward than most car rental firms. I can also recommend their customer services after having had a problem!
When we got to the car it wouldn't unlock. We thought that given it was Sunday morning we were in trouble but turned out that the phone number we had had a very helpful and apologetic person on the end of it. They managed to change our booking remotely to one of the civic centre cars and offered to pay for a taxi for us to and from the civic centre for us.
Once that was done it all went very smoothly and we found it really easy to work - refuelling using the card in the car was also straightforward.
Cost wise it's massively cheaper for us than owning a car based on the amount of usage that it will get and it's a cost we can easily cut if our finances were to change by just deciding not to travel rather than a cost we have to pay every month. It is slightly more inconvenient than having a car on the drive of course and for longer rentals for example weekends away and holidays it is probably still going to work out cheaper to rent a car from a rental company. The flexibility is great though and the inconvenience probably is good for the environment as it makes it less likely we'll use it just because it's raining or similar!
We'll be keeping on with it and would recommend it to anyone who needs a car just on the odd occasion or a second car occasionally - there are locations at Kennet Island, civic centre (just off castle street), the avenue school and Kings road