Yahoo is blowing their chance. Put it on my high pageview page, used to 15-20% CTR, best day over the week was 0.8%, needless to say targetting sucks, ads appear to slowly from initially putting them on the page.
All they had to do was copy google and pay more and they had the market. They are making some huge mistakes right off the bat.
On the very same day, in fact it was almost the same hour that I got an email from YPN inviting me to join their beta my AdSense earnings almost doubled. Coincidence? Actually I think it might be, but that sure did discourage a switch. :)
Same thing happened to me. Got an email from Yahoo telling me I could sign up with their Beta program. That same day I had my best Google payout ever. It did make me wonder since it quickly went down to normal the next couple of days.
I did try Yahoo on some pages for a few days out of curiousity but their targeting and ad supply is just terrible for my sector anyway. If you had a more general type site, it might work pretty well for you. I was getting a terrible click-thru rate but the clicks did pay well.