While it is not a unique system, It is a Good Thing because it is backed by an industry leader (Amazon.com). Since the Banner Ad market has recently gone flat, micropayments remain one of the few way for "free" service to stay "in the black". I can't say I like the idea, but if it's all we have...
My Problem with the system thusfar is the amount of the payments, which is at around one dollar. One US Dollar is still a lot of money for me, and at my current pay scale takes me 10 minutes to earn. Is it fiscally responsible for me to pay 10 minutes of time to use a page for only 30 seconds? Now, if it was only a few cents, I'd be tempted to do it more often.
However, there is one point that I cannot stress enough: If you plaster the feces that is is the Amazon Honor System all over your page, get rid of your banner adds. Having one or the other is a way of breaking even, but having both (especially in close proximity) is just very, very tacky.