http://www.nytimes.com/2012/04/14/your-money/relayrides-accident-raises-questions-on-liabilities-of-car-sharing.html?pagewanted=all$1-million may not be enough. And traditional insurers are considering cancelling policies of insureds that they find are renting their cars in this way.
If you're a judgement-proof 20-something with car payments to make, this might seem appealing. But if you have any kind of net worth, I'd steer clear of this kind of thing.