I run Adsense and YPN on the homepage of my main site interchangeably to test which is better (equal pageviews at the end of the day, same weights). Both ad networks are both on target, although Adsense ads show more "less sophisticated" adverts (no brand name, mostly mom and pop operations); while YPN shows brand names. Google’s Adsense gives me double digit CTR, while YPN only gives me 1/10 of Adsense’s CTR. At the end of the day, even if YPN gives me higher earnings per click, Google gives me better revenues. It seems that G’s targeting is designed to show what ads fit the page AND what ads are most likely to get click (probably due to some historical data of my target audience or whatnot). On the other hand, YPN only shows what ads can possibly be targeted to the content, but not really designed to generate high click thrus.
This is a key question for most Web 2.0 companies relying on advertising as one of their main source of revenue. At the end of the day, the depth of the advertising network and the more effective relevancy algorithms of Google seem to be the main differentiator leading to a substantially better yield.