I started with AdSense in July, 2003. I believe that was the second month of its existence.
Here we are 4+ years later and it's interesting how common it is to read remarks about people deciding whether AdSense is worth having - as compared to the initial months/years of AdSense. I don't recall reading those kind of remarks often in the beginning.
With YPN it seems to be just the opposite. It's still in beta and people often have to consider whether it's worth having on a site.