Not a bad adaptation, with the usual to-be-expected differences that arise when an adaptation is being authored as a film is in production. The setup (everything leading up to the chestburster) is what's expanded the most, taking up a good 3/5 of the book, while all the action goes by pretty quickly. Also interesting (to me, at least) is that aside from the facehugger, the alien forms are obviously from before H.R. Giger became involved in the film, as the chestburster uses the early-in-production "butchered turkey with teeth protruding from the stump of a neck" description, and the alien itself is barely described at all, beyond "big."