While I have been using Google ads on the blog for some time (and they do quite well), they don't yet do RSS ads outside of a more limited beta program. So I'm trying Yahoo's ads, also in beta but I'm on the list.
They just went live, and all that's showing right now is a generic ad, presumably until they spider the site and figure out what ads to run. Ideally it will be ads as relevant as Google Adsense does.
Competition between Google and Yahoo will be good for publishers. Just on basic click-rates, one will tend to do better than the other, presumably. If one is consistently doing not as well, they will lose all the partners, who will flock to the other. The only way to fix that will be to increase the percentage of the money they pay out, until they get to a real efficient market percentage they can't go above.
Read on for examination of the economics of RSS ads.