I played some demo of an Aliens game. Very short sight range, so the Aliens really seemed to just kind of pop out at you. You know, they just sort of materialize out of the darkness in front of you or the steam venting from a nearby rupture, and pretty much eat you whole if you can't back off and get out quickly, while giving them something to chew on to slow them down. Then the face huggers get up close where you can't see them and are never sure where they are or where to shoot. When you're new to fps, they can really make you frantic with the keyboard and mouse.