I had a brief 2-3 month affair with Yahoo, when the paid out well, but now I'm back making more with Adsense.
I'm almost totally back to Adsense, except for one page on one site that seems to do much better on YPN, even though the ads are totally irrelevant. It gets about $50 a day on YPN and about $5 on Google (with relevant, but low CPC, ads).
The relevancy with the Yahoo ads is not nearly as good as AdSense, but they per-click payout has been much better. I've taken several sites with low click rates on AdSense and switched them to YPN.
Yahoo seems to go in and out for me with good ads and bad ones. On the bad days it does terrible. On the good days, it's done great. Google is stable with decent ads.
Yahoo is between x3 (worst case) and x6 (best days) of what Google used to pay me. Sometimes I run 50/50 briefly and always, yahoo beats google. I dedicated pages for adsense just to keep that account alive in case YPN disappears. I am all yahoo at least for now
I make about 5 times more with YPN than with Adsense. So when I have an off day with YPN I just take it in stride.
I still use adsense on a couple sites because the ads are more relevant and pay better then YPN. You have to play with it some. You might find that some ads from one does better on a certain page of your site and so run YPN on one page of your website and Adsense on another. It is a matter of experimenting with the results.
"You might find that some ads from one does better on a certain page of your site and so run YPN on one page of your website and Adsense on another." Absolutely. That's what I do. Yahoo pays zilch on one page, and G pays well. G pays zilch on another page, and Y pays VERY well. Periodic testing is required, since everything is changing frequently. Hard work, no doubt about it, but it maximizes income. BTW, I make more with AdSense, because of consistency of ad delivery. Sometimes, YPN delivers good (targeted) ads, and then promptly loses it's mind, and goes totally random. I don't make good money with random ads, nor do I want to display them, because it makes me look like an idiot. Because of this, most of my high-traffic pages are AdSense, except for the ones for which Google does not serve good-paying ads. I moved my low-CTR pages from AdSense to YPN, because G likes high CTR. Since YPN is low CTR anyway, there is no penalty, and sometimes there is a surprising improvement.