I didn't get a chance to read King's original short story so I won't be able to compare. However, I'm a big Lovecraft fan and had quite big expectations for the film. The overall plot is solid, albeit with some stupid logic holes (harhar let's put more lights in front of the shop windows, because clearly they attract the alien bees and I like bees). The theme of people being isolated with the outside world being full of invisible and otherwordly danger, and the inside becoming dangerous as people start to get divided and get violent is of course an interesting one. The execution however is a complete letdown. The film fails to deliver any scares or horror or any proper tension. Oh and, the ending? Anyone who's read Lovecraft's "In The Walls Of Eryx" would have seen it coming from a mile afar.
Overall The Mist isn't really bad, but it's painfully mediocre.