It wasn't a great movie. The mist arrives too quickly (before I've decided if I like the characters involved) and the film in general has the feel of a "straight to DVD" film about it (i.e. it was only given a theatrical release as an after thought). The one saving grace about the film was the twist in the tail that this film has. It was predictable if you've seen any horror films in the last 10 years, but the fact that they went for a "non-pat" ending anyway impresses the pants off me.
Incidentally, I wish someone could tell be the difference between a novella, a long shirt story and a very short book.