Federated used to be pretty exclusive...not any site would qualify. They acquired Lijit last year which runs CPM Real Time Bidding ads. Whichever one you got invited into it's probably worth a test. Pay attention to your page load time with Lijit...it used to be a real drag.
Tried them for a few weeks now. Fill rate is quite low. However, i have high traffic sites so revenue is not so bad which actually pays off my monthly hosting bills. Adsense still reigns supreme.. not even close.