Don't bother buying it. The singleplayer is short and unsatisfying, and the AI is very stupid. The multiplayer is unbalanced, and the community is pretty much dead at this point (for the PC, anyway).
The combat (and the rest of the game) is buggy and irritating, and all of the melee combat is auto aimed, without any option to turn it off. There are only a few weapons for the marines and predators. I wouldn't consider the stealth kills to be bad if they weren't so buggy. Speaking of bugs: Rebellion takes a long time to fix a few problems.
It also needs Steam, in case you don't like it.
It's okay for a budget game. There's a reason the PC version was $20 at release.
Where did you buy your copy? It cost me $49.95 at Amazon; I would probably feel ripped off even if it cost $20.