We have them on our sites. They have a very good conversion in the sense of sign ups but not in depositers. They have this freeroll offer that you can play without needing to deposit or even register a card.. I played myself for a while and managed to build up a little bit of cash and this I could then use to play real games with.
Imagine that 99% of the players do this and when the money is gone, the simply restack their accounts at the freerolls.
Other than that.. have not had any problems with them and they seem to be pretty steady.