I started Stephen King's novella but didn't finish it, so I can't say how faithful Darabont was to King's original material. What I'd read was recreated well on screen. However, the film lagged at times, especially the scenes with the religious nut, played annoying well by Marsha Gay Harden. Also, there were several unintentionally funny scenes. Given what was happening in those scenes, I doubt the director was going for laughs.
So, we have a horror movie that dragged in places, was laughable when it shouldn't have been, and wasn't as scary as it should have been. In spite of all that, I found the film disturbing.
I don't want to give anything away, so I'll just say I enjoyed what was in the mist and was majorly creeped out by it. I also liked the brief explanation of it.
I didn't hate THE MIST, but I won't tell you to run out and see it.