If you haven't played it first time around you owe it to yourself. It has more pants peeing moments than any other shooter. The single player as marine or alien is totally stressful because you die really quick and there are no fixed spawn points. To the marine every alien is superfast and superfatal. There were no fixed light sources and, at least before gold, no save points.
The original perfectly captured the feel of the movies, AVP2 was a pale comparison. When I attacked people with the alien my friends would ask each other "Where did he come from?" and the answer that I most heard was "Everywhere."