A FEW MINOR SPOILERS BELOW, IN RED:
If you totally forget the premise of the game, it's a pretty entertaining movie. There are no demons, it's merely a special gene that makes you very strong and regenerate quickly, but if you happen to have the evil gene when you are injected with the gene you turn into an imp or some such creature. It's not very badly written, though the story has been done many times before. The FPS sequence near the end (inspired by the game, of course) makes the price worth it alone. If you rent the movie hoping for awesome weapons like the plasma gun, rocket launcher or chaingun, you'll be disappointed, because they do not show up. The BFG has two or three throw-away uses.
EDITED TO ADD: In the special features, Wayne Johnson (The Rock), during training, strongly praises the military, and though I can't remember exactly what he said, he basically said that the military was much better and braver than he. I thought that was cool.