I've just started playing around with this and it seems like another awesome tool. I'm not sure how to fully exploit.. i mean utilize it. Using the site target feature, and ranking sites for a particular term, you can see who gets the most traffic.
Might be good for quality content/theme ideas, potential site purchases, direct advertising negotiations, what else? Seems like a host of possibilities to uncover.
Would it be safe to assume that sites that have Adsense as a revenue stream are mostly individuals not selling products themselves, but just trying to farm out traffic... Probably more than a few subscribe to this board.