Scott Le Vine, Research Associate in transport systems at Imperial College London, and Board member of CarPlus Trust in the UK, is one of the most thoughtful people about carsharing ("car clubs" in the UK) I know.
He has written the best thing I've read about the Avis - Zipcar deal - published on his Planetizen blog here. Scott does a great job of taking a look at the big picture of carsharing and future mobility.
Well worth your time.