I've seen it, I thought it was ok. Quite silly and gory. The scariest bit of all is near the start when they are caving. If you like films like Dog Soldiers etc then it might be your thing. Have you seen it?
I won't spoil it for you...but The Descent contains one of the biggest 'jumps' I've seen in any film this decade.
Other than that, I thought it was pretty effective as a thriller/horror film. It's very gory, and it helps if you suffer from claustrophobia (my partner does and said that the potholing scenes were worse than the 'truly' scary scenes).
By the way, if you understand the ending, let me know, because I still don't have a clue.
I suffer only a tiny bit o claustrophobia and the scenes where one of them gets stuck a little scared me more than anything else. Rather enjoyed the film 8/10