My one blog got approved for their ad services and I used them for about 3 years. You have to meet their requirements in order to be a partner for their ad program. Anyways, I was earning good money in the beginning and it seemed to grow pretty fast. As time went on, I found them to actually be slowing down my site and I was losing my ranks because of it on Google. I ended up switching back to Adsense for this reason.