There's a demo out. Right now only preorders (I preordered) can play it but in a few days everyone will be able to.
It's pretty cool, actually. There's two missions, a long one with no surprises and a shorter one where you get a jump pack. You get four weapons, the standard bolt pistol/chainsword combo, a bolter, a stalker pattern bolter (Sniper rifle, basically), and a vengeance cannon (Or something like that), which I'm pretty sure doesn't exist in the actual fluff. It's just a sticky grenade launcher basically.
The ranged combat's okay, not much to it, the melee combat's fun, you get covered in blood, the finishing moves are cool (When you kill a nob you knock his choppa away and shove your chainsword into his mouth), you really get to feel like a space marine. You have a healthbar that doesn't automatically recharge and an armor bar that does. When the armor's down your power armor sparks and whirrs and shit, which is a nice touch.
The jump pack's cool I guess, not a huge deal. You jump high and far, you can ground pound and kill people. No big deal.
[spoiler=Sperging out]
They did it again. Bolters are machine guns. You can fire your whole clip in one burst instead of being limited to four round bursts like an actual(hurr) bolter. They also have shell casings and don't explode. Typical.