Theatrical cut is miserable as hell, see the extended version.

Rating: 4/10 (22 April 2001)

In regards to the version that was released in cinemas...miss it. That's for damn sure. This was the first film that David Fincher directed and it has that dark, smelly and grimy look that he is only capable of. After the highly exciting Aliens there is ZERO entertainment here and only one Alien. Very fake looking and largely unseen for most of the movie.
Apparently the Alien Queen laid one last face hugger egg on the Marine ship (didn't she already detach herself from her birth tunnel thingy). The egg hatches and lays another Alien egg in Ripley, which just happens to be a queen. For a reason, which is unclear to us, the face hugger sprays acid on the ship, which catches fire and breaks off the cryo-tube section. This sends Ripley, Newt, Hicks and Bishop crashing down onto a lice-infected prison planet called Furiona 161.
Just for the hell of it (it seems) Newt, Hicks and Bishop are killed off because the film is deliberately miserable as it possibly can be. Weaver is the only female character in it and is very much indistinctive among the rest of the indistinctive cast as she is bald-headed (because of the Lice problem) with a bunch of bald-headed guys who all appear to be English and ugly.
There is another face hugger (so the Queen laid 2 eggs before it died?) that the prisoners seem to miss as they rescue Ripley. It impregnates a dog and a new Alien is born in the form of a canine, giving us the impression that the beast mimics the form of the creature that became its host. We are supposed to observe this as it is never cleared up in dialogue.
There are numerous scenes in which Ripley (or bald-headed prisonersI can't tell) run down damp corridors (or perhaps the same one over and over) being chased by the Alien. But for budget reasons we don't actually SEE the Alien, just its point of vision. Characters are killed with no care, passion or meaning to their deaths and how the hell anyone believed that an audience of any kind would enjoy this film is beyond my great understanding.
It is so bad that Fincher didn't get work for 3 years after this until he was called upon to direct Se7en. The only reason he got to do that is because it is a depressing and ugly film, much like this.
The extended version featured on the Special Edition DVD is a far superior version of the film, though not a director's cut or without faults, it's the best version of the film you're likely to see. Characters are fleshed out, sense-making scenes are restored and the creature is born out of a dead ox instead of a dog. It doesn't sound like much but it almost changes it into a completely new and more satisfying film.
Either 1/10 or 7/10 depending on the version you watch. 4/10 overall.