As a general rule, I don't tend to like horror movies, unless they're exceptional. ("Alien" and "The Sixth Sense" would top my list there.) I don't go near slasher crap, and I much prefer suspense.
"The Mist" is Horror with a capital 'H', but I watched it because it was Frank Darabont adapting Stephen King (plus, there seemed to be a bit of a sci-fi element to it), and I was not disappointed. In fact, I was totally blown away. This movie grabbed me by the jugular from the first scenes and didn't let go.
Even with all the terrifying things coming from the mist, the greatest horror the characters face comes from each other; and while many people complain about the ending, to me it was incredibly effective, and absolutely devastating.
If I have any problem with "The Mist", it was with the uneven CGI--at times brilliant, and at others too obviously fake. But the story hooked me so much, none of that mattered. A masterpiece.