Played the demo and it seems to have some visual bugs. For reference I have a Dell 518, Vista Home Premium, 4870, latest drivers.
When you first start the game, there are some waterfalls/mist effects going on. Some of them work, some look like big orange squares. At another point, you can look out over a landscape from a cliff. If I look straight ahead, it is looks like mountains covered in fog. However, if I turn just a little bit to either side, the shapes of buildings pop up.
Also, the indestructible mode (forget what its called) that happens when you collect enough energy/souls from the bads guys you kill, is completely unusable. It is hard to explain what the screen looks like, an extremely grey/blurred image with no recognizable detail, but the end result is you can't see a thing. Once the effect is over, the image returns to normal.
The game engine itself seems rather dated. Enemy AI is quite bad as well. There were a couple of spots where I was firing at enemies and they just stood fast and did nothing, I took a couple of steps forward and then they suddenly started firing back.
I going to pass on the Steam preorder. Maybe the demo is an earlier version of the game. Hopefully the full version won't have the visual problems. If it does, I will wait for a patch before buying.
-KeithP