it's not perfect but I think it's pretty solid for a horror movie. The film has a great grasp on tone, and the sets are amazing. Simon Bowles did a fantastic job as the production designer, and the sets they use, combined with the way they're lit, could fall almost anyone into thinking this is a real cave.
What I like about The Descent, is that it's not your typical horror movie. There's no cheesy love story going on with underdeveloped characters you care nothing about. Believe it or not, there are actually subtleties in the film. Horror movies use different tactics to scare you, and a lot of them use the same ones over and over, but I thought The Descent was pretty versatile. It's not just a movie filled with, "Boo! Ha ha! You were startled."
The cave environment brings on a feeling of claustrophobia, the lighting is perfect for a horror movie, and they have the right excuse for it to be dark. The cinematography is well done, it has a good score that helps set the tone without forcing it on you, it achieves what it set out to accomplish, and that's the most important. Again, it's not a perfect movie. Sure, a girl has a conversation after being stabbed in the throat and the characters could be a little more rounded, but I think Neil Marshall proved himself with this movie in some way. What I love about Neil Marshall, is that I know that he enjoys what he's doing.
