It is not only the best horror game ever made, but also is among the best sci-fi games to date. The sterling reputation of the Xenomorph's AI is well-deserved, and the attention to detail in the graphics, sound effects, art direction and musical score all directly fall in with the original film – to the extent that it feels like you're actually playing the film at some points. Creative Assembly has managed to truly capture the essence of 1979 ALIEN, not only in the tension and horror that encountering the Xenomorph brings, but also in the nuanced and unique sci-fi aesthetic of the ALIEN brand.
It's been out for 7 years now and it still holds up incredibly well, it doesn't show its age much if at all.