Ah, "The Mist" is one of my all-time favorite King stories. I still remember bringing the Dark Forces
anthology home from Harvard Square and reading it in one sitting. Sigh. Those were the days.
The movie was easily the best adaptation of a King story ever (which, granted, isn't difficult) and I love that they included a black-and-white version on the DVD. I remember a long-ago King interview where he said that he envisioned it as a grainy B&W monster movie while writing, and it really fits.
I'm not sure why they decided to change the ending when the rest of the movie was so faithful, and I would have preferred the original, which left the characters with a little hope. That said, it's not really inappropriate for that kind of movie; and I wouldn't really describe it as nihilistic, since the horrific irony of the personal tragedy is balanced by the optimism of the apparent end to the overall peril. Though this group of characters have met with disaster and despair, mankind has been saved.