Demo is out for Steam preorders. If you go to spacemarine.com and sign up for the Honor Guard, you can get Steam access now as well. Xbox demo will be out sometimes tomorrow (Tuesday).
The game is awesome! I thought it was going to be a lot of shooting, but it's really more melee combat than anything else. Lots of chainsword action, kicking ork heads in, and splattering blood everywhere. Basically you beat the shit out of the enemies and then when they're weak enough, you can kick them to stun them. Once an enemy is stunned, you can execute them and get some health back. So you just wade in ripping the enemies apart, and then start executing if you get low on health.