I'm in love with searchfeed at the moment. I have a site with mass appeal that wasn't even designed as an arbitrage site but its working as one! Penny bids result in 100%-200% ROI. There aren't that many clicks coming in so it isn't big money ...but it rolls in every day.
I also tried out adbrite but searchfeed has been much better. The users don't seem to hesitate to click on ads.