I watched it last night with my wife. She didn't realise that it was a horror film (I chose it so she blames me). She was so terrified she had to watch Star Wars afterwards as a way of settling herself down for sleep.
It's an interesting movie. Claustrophobia is played well, with one of the characters getting stuck in a very tight cave tunnel. I thought the "Carrie" impression at the end of the film was overdone, though. As was the brutal depiction of a compound fracture of the leg.