The thing with Aliens is, it was made when CGI wasn't even thought about. CGI plays such a huge part in any film that gets release nowadays that if the effects suck, people won't even watch it!
I have seen Aliens so a hundred times, and every time I see it I don't have to make excuses, (well obviously thats a green screen; that's obviously not real but OK, I suppose it's technically possible (James Bond I'm looking at you!)).
Aliens has to be one of the best films ever made because it didn't rely on that! Cameron managed to make that film so intense that you don't get a chance to breathe until the credits roll, and by the time you try to reflect on it you find yourself back at the cinema for round 2!! And there was not 1 thing in it that I looked at and thought "No way"
Suspention of disbelief is one thing, but it is so much easier when the film do's it for you!