Yeah I played the demo. Dark Eldar weren't bad, I dunno what you're talking about. I kicked space marine ass in the demo! Hardest part was building up at the start of the skirmish map, since the computer was set to a pretty high difficulty and while wassnames dark eldar's "scouts" could cream SM scouts and put up a good fight vs Tactical SM squads, they just weren't good enough to beat the Tactical SM rush the computer always did, so you had to rush yourself too. The campaign map was easy peasy.
EDIT: Oh yeah and while I am pretty happy at the thought of 2 more races to control and conquer, I'm not as excited as I was with Dark Crusade, because Necron and Tau kick ass, especially Tau. FOR THE GREATER GOOD!