Part of the reason I won't buy expensive apps on iPhone is because there's no recourse if they're terrible. Google's 15 minute trial period is a start, but for some games that's not long enough to download the content they need to run.
It will also motivate developers to create apps with more than 7 days worth of value. How many people will actually keep a fart app or a game with 2 levels for more than 7 days?
I think this will be an interesting experiment and look forward to seeing what effect is has on the quality and quantity of available applications.