SpaceRef recently started featuring Google's Ads and although they aren't as optimized as they should be yet, they will be when I get back, the results have been pretty good. Interesting to note that our web site Commercial Space Watch gets the highest clcickthrough rate. The following article from Search Engine Watch is worth reading.
Goodman Revisits AdWords Changes & Importance Of Clickthrough: "The Importance of Clickthrough Rate (CTR): Has it Changed? from Andrew Goodman at Traffick is a very long look at recent changes to how Google is making use of clickthrough rate as part of its ad system. He highlights that AdRank is no longer CPCxCTR but instead takes in some other factors such as historical performance or ads with certain unspecified characteristics. In short, it's CPCxQS, with QS being a quality score made up of some things an advertiser can't visibly see. So an end to the importance of clickthrough? Andrew hopes not, seeing clickthrough as still a great indicator..."
(Via Search Engine Watch Blog.)