As for the movie, I thought the whole Octopus/Bug/Monster thing was a little cheesy. The story could have been a lot more tense and mysterious if they left it at The Mist..a state of unknown fear.
Those parts are actually right out of the story, probably some of the most literal parts of what King wrote.
The problem, really, as with much of King's stuff, is that what works really well on paper (because he knows how to weave a narrative) visually does not work well. He's a writer, not a visual artist, and much of his stuff has not translated that well. The better pieces are the ones that focus on psychological horror and stuff that doesn't use a lot of special effects.
In the story, the tentacles, spiders, etc., were really creepy in how he wrote them. He's quite the audial/textual magician, he can take crap and make it look good on a page... but don't expect it to translate well.
edit: I REALLY enjoyed the ending. That is a horror movie ending, and it is definitely appropriate in light of King's psychological terror sense of consciousness in his works. That is exactly how a horror movie should end in my opinion.
King's original ending wasn't anywhere close.
Darabont usually does "feel good" movies too; I took it as his way to give everyone the middle finger and say, "Hey, look, I can be dark too!"