Either without Ripley it is not perceived, or the actors' reactions to what happens are too unrealistic - it feels like such monsters are attacking them in the order of things; when they left their friend just lying there with an alien on her face, I didn't get scared - I smiled.
So halfway through the film, I finished this Alien a'la Rome.
If you took out all the actors, I think I'd probably finish it because of the level of the sets and the filming.