I use Zipcar occasionally and find it useful as their cars & vans are all over London. You can cancel up to 3-4 hours in advance of your booking with no fee.
However, sometimes they change your reservation with 24 hours notice so you have to keep an eye on your email. For example, I had booked the Zipcar Golf on Bermondsey Street on Sunday 17 May, but received an email on Saturday 16 May saying that this car was unavailable and they had booked me the Golf at Druid Street instead. Good job I checked!
Okay (at best) for infrequent use. Concept is good but execution is lacking - mainly due to other members not being considerate in getting the car back on time, etc. Customer service substantially deteriorated when Zipcar bought Streetcar. I stopped using it not too long thereafter, after an incident which is described elsewhere on the forum. City Car Club is better but less cars.