Really depends on the site. I saw my earnings almost double once I added YPN along with Adsense - on different pages for sure, but I don't like their lack of ads. It seems to be very true that my number of clicks on Yahoo are down percentage wise compared to Adsense, but some click are paying 10 times what Adsense is. I'd target a few pages and just watch them. I watched my Adsense channels for over a month, picked out my low earning pages and gave them to Yahoo.