A difficult movie for me to judge. The setting was beautifully done: the spacecraft, the Alien planet, steam hissing, slime dropping; the overall uneasy/scary feeling was great. But the characters running straight towards the danger annoyed me, the shock-moments were too easy and the biggest problem: I've seen too many movies that try the same thing, so it doesn't really work for me anymore.
Two pro's for Alien though: (1) I believe it was one of the first space-horror movies and (2) even though there have been many copycats, I don't think any of those copies have actually improved on the original Alien. I just wish I had seen this one first :)