I don't think you can signup with them directly. They serve their ads through Adbrite and ValueClick (and probably other networks, those are the 2 I know of). They are CPM ads on Adbrite/Valueclick. Using them presents a rare opportunity to serve relevant text cpm ads. I like their ads, but do not use them. I'm holding out for a while to see if they can be trusted. I know that they do slip a graphical ad in every now and then, and you really do not have any control over those.
I agree with Corey. I'd just go with Adbrite since it's the big player out there although it's more costly than adengage and smaller players.
Adbrite is definitely the most solid alternative to Google Adsense. Start using them as soon as possible and you will see some decent returns.