So this isn't really post-apocalyptic in the same sense as The Book of Eli or The Road, but since the movie's main conflict arises from a fear of the end of the world (and, you know, giant Cthulhu monsters) I think it qualifies.
I watched this mostly because I was interested to see how the director, Frank Darabont, handles a horror piece. Given how well directed this movie is, I'm pretty eager to see how his television version of The Walking Dead turns out.
Anyway, The Mist pretty much Stephen King by the numbers. The acting and directing make the film engaging and adds some tension to the plot, but its all material that has been done before. The focus on fear (in this case fearing of an impending biblical apocalypse) adds an interesting layer of drama to the story, but the writing isn't strong enough to support it and it ultimately peters out by the end. Had the writing been better (and really, endings have always been Stephen King's weakness) this film could have been something really interesting. With that said, the direction and acting in the film is good enough to make it worth seeing.
The film is becoming increasingly famous for its final scene. While I think it was a bold artistic choice, I'm not sure how well it fit with the tone of the rest of the film. I don't want to spoil anything, but it seems to me that ending the film like that leaves the major conflict of the film unsatisfied. I'm perfectly happy with ambiguous endings, but when you have a film like this end in the manner that this film does, it ends up completely overriding everything that came before. So, while I think it was an interesting way to end the movie, I'm not so sure it was the right way.