Here's a rough comparison of the services. Note that I have used both.
Enrollment: Both have applications, but AdSense is immediate (or almost immediate) whereas Yahoo takes 2-4 weeks (in general) and many people are rejected (remember that it is in beta).
Targeting: I'd rate AdSense at 8/10 and YPN at 4/10. Note that this is not always a bad thing. If you have a site that is in a very low paying niche, then having slightly offtarget ads is greatly beneficial to you.
Earnings: My CPM on YPN is about 5x what it was on AdSense. A couple of notes on this though. First, they are in beta and changing things around. The last few weeks have been very, very bad for me with a cpm about 1/2 off what AdSense was. It has picked up the past couple days slightly, but it is troubling. As with all things web related, it comes and goes. Second, it is in beta and the earnings are going to drop. Right now they are probably giving close to 100% to publishers to provide an incentice to stay with the program and to give them good PR. This *will* change.
If anyone has any questions, feel free to PM me.