I am testing it in one of my smaller sites as well as in the homepage of my main site. In my main site's homepage, I have it alternate with adsense using the same weights so they have an even number of impressions at the end of the day. For the same prime spot (this is a leaderboard and both uses the same exact color scheme), G gives me double digit CTR but Yahoo gets me only a tenth of my adsense CTR.
While YPN may have a higher cost per click G's ads are more suited and attractive to my users. YPN 's ads are also targeted, and in fact the advertisers are bigger and more sophisticated than adwords advertisers, but my users respond more to adsense than ypn.
I honestly think it is the layout of the ad. YPN ads are only displayed in part. Adsense ads are totally visible...no guessing. The same YPN ad also appears on the next page, and the next...no variety.
Yes it is in beta, but I would hope to see these situations fixed quickly. Those participating in the beta will only stay as long as they see SOME benefit. We put it on our smaller site, the ctr is way down, but the clicks are worth more....which means it's a wash. If I had to pick one, the more visually appealing, better targeted ad wins.
I'm patient though...we don't have it on our big money making site so we can watch and wait for a while.