Interesting idea, biggest drawback I see is while most people may only use the car for a few hours a day, say commute to and from work, the time they need the car is the same time everyone else does. It would work for those that only occasionally need a car say to go shopping once a week or for a vacation trip a few times a year, primarily city dwellers who use public transit, walk or cycle to work or work from home. I think that market is currently using car share services like MODO or EVO. Course if the idea takes off, there will be fewer cars needed, good for the environment, not so much for car manufacturers.