Full tilt is, in my opinion, one of the best sites to promote. They convert like absolute crazy, ask any serious affiliate of theirs and you will see amazing convertion rates of 20-25%+ and even up to 45%. They also do a great job of retaining players with their mass marketing and promotions.
The only weaknesses they have are mainly a sub-standard affiliate control panel, non-personalised ‘affiliate managers’ and of course the fees. But the way they convert more than makes up for it.