Wow. This game will creep you out as much as you will ever be creeped out by a video game.
The whole first marine mission (which I finished a little while ago) gets you used to false readings on your motion sensor. Little moths are flopping around, cranes are swinging in the breeze, then all of the sudden WHAM! when you walk into this room your attacked by some fast little bastard aliens.
Then you're expecting an attack, and they're playing this music that sounds just a little like aliens screeching.... so the whole time you're paranoid, looking behind you to make sure there's none there (since the motion detector doesn't detect motion behind you).
Of course, it's dark, and all you've got for light is your sholder lamp and some flares.
BTW, for anyone who played the first game, this game seems to let you survive a little longer, one alien doesn't kill you.
I'm giving too much away though, for anyone who wants to play it!
The engine is much improved in this sequel, too. Actually it's a whole new engine. Requires a beefy video card though, and lots of RAM. To run the game in high detail, they recommend you have a 700 mhz cpu, 256 MB RAM, and a 32 MB graphics card. Seemed fine on my computer. I have a 700, 192 MB RAM, and a 64 MB Geforce 2.
If you find the 2 CD ISOs for this game anywhere, and you have some time to kill, get it!