Truly horrifying, truly grotesque, and completely impressive.  I feel like I have seen movies like this forever...movies where an alien life form can take the form of humans, and humans become increasingly paranoid about each other.  Never before has it been done so well as in this film.  The make-up effects could be cheesy and silly in a lesser movie, but John Carpenter makes them nightmarish.  I can't believe I had never seen this film before.