I watch this film at least once a year. I first saw it around 6 years ago when a flatmate suggested watching it. I'd never heard of it and, seeing the title, argued not to watch ( I was probably thinking of how bad The Fog was) but that would have been a mistake!
This film really surprised and enthralled me. I was stunned at just how good The Mist is.
The characters and the set up all work so well (Toby Jones standing out as usual!). The supermarket that all the characters are trapped in becoming a metaphor for all that society contains, both good and bad. The resulting conflicts are fascinating and often darkly humorous. Even the first quarter's dodgy cgi doesn't affect the atmosphere of impending doom and destruction (later shots are very impressive). The atmosphere and tension are so well maintained throughout, building up to a climax that is probably one of the most shocking ever filmed.
If you like dark films with a bit more to them than the usual horror, check out The Mist.