One of my complaints with this movie is that it takes longer than it should to really get going. But part of that is a bit of misdirection. You think this is going to be about Sarah and her overwhelming grief at losing her husband and daughter in a car crash. Then you think it's going to be a standard cabin-in-the-woods movie. But when these chicks drive out to a cave and the guidebook gets left in the car, then you know where it's going. In part. :)
The first part of the cave adventure - before the really good part - kind of infuriated me. One person does something really stupid - leaving the guidebook in the car to make the trip more exciting - that risks everyone else's lives. It was like Mike in the Blair Witch Project throwing the map away because it was frustrating him and he couldn't see the use of it. I mean, DUH!
This would still have been a fairly effective movie if it had just been about these women going spelunking and getting lost and hurt in the dark, possibly with no way out. But this movie kicks it up a notch. There's something out there in the darkness, hunting them and eventually killing most of them in a spectacularly gruesome fashion. You see them panic, turn on each other, figure out how to outsmart the creatures, and make unthinkable decisions. It's incredibly tense, and I spent much of the movie with my fingers in my ears, like a little girl.
The ending is somewhat confusing. Did Sarah dream the part where she gets out and just decide to stay down there in the cave? WTF? But the ending aside, this is a pretty amazing horror movie.