I'm making money as a YPN publisher. The key (if you're thinking of moving from AdSense to YPN) is to switch those pages over that are performing poorly with AdSense. Look for low CTR / low EPC pages and put YPN on them. YPN EPC is much higher! However, YPN CTR is much lower (in most cases). So you only want to try it with pages that are currently not getting good CTR or EPC with AdSense. Basically, you'll be squeezing out all the earnings you can get from a page that isn't making you much right now. If you switch pages that are performing well with AdSense, you may regret it. I know I have in almost every case. Once I figured out that YPN is best with the poorly performing pages, I found a good mix of AS and YPN. The bottom line is it increased my earnings.
I'm trying Y on all pages.. if it beats A then it stays, if a tie then Adsense wins because of Y's delayed payment.
I'm finding what is said above about low EPC/CTR pages is true, but I'm also finding many very high EPC/CTR pages are doing VERY well when the targetting happens to work. When targetting is working I'm often getting 3 to 4 x better EPC with Y.
The more I test it, the more I can firmly say that YPN is a good alternative for pages that you're getting low CTR with AdSense. I have yet to strike out. I'll also say that if you a site where just about every page is targeted to a specific keyword or two, it can pay to switch the entire site to YPN. The key is to compare CTR (AS vs. YPN). Go slowly at first and if you are getting similar CTR's, I'll bet you'll find that YPN blows AS out of the water. Much higher EPC.
I agree with the theory of using both G and YPN on your site. I also use YPN on pages that I was not previously running G ads due to poor performance. Using the Ad Category feature I am able to tell YPN some good ads to display that my visitors might like on those pages. Seems to work out well as I have almost matched my Adsense income with YPN and didn't have to replace a single G ad.