It took me almost a month after its release, but I finally saw the film version of The Mist a week ago. I liked it, but it wasn't a classic or anything. It certainly wasn't scary (when are filmmakers going to get it through their heads that CGI effects don't work in horror films???) but it was fairly tense.
In terms of faithfulness, it followed the book pretty closely, which may have worked against it; i.e., since much of the action takes place in a grocery store, it gets pretty "stagey" after a while (is that even a word?). And in that respect, it certainly doesn't help that Frank Darabont used crew members he met while filming an episode of The Shield - the movie often seems like a padded-out episode of The Twilight Zone or The Outer Limits. You'd never guess this was from the same man who made The Shawshank Redemption.
Finally, if you haven't heard about the ending... well, I won't give it away here, but I will say I was very unhappy with how the movie ended, and many of the reviews I read said the same thing. It's just wrong. That's all I'll say.