I saw this with Kyle and Joe yesterday, and we all thought it was pretty good. Plotwise, I had no idea what to expect once they got down in the cave, and I think that made it a much better experience. There were some truly terrifying parts, along with some cheap thrills like having someone suddenly appear next to them or something like that. The pacing was excellent; once it got going, it didn't let up. (It did take about thirty minutes before it got exciting though, and everything that happened in those thirty minutes were utterly worthless.)The characterization was archetypal, for the most part. Where it strayed, it was even worse: the main character was unlikable, vindictive, and cruel, qualities you don't really want for the hero of a survival horror movie. The acting ranged from subpar to pitiful, but you aren't going for the acting, are you? All in all, a pretty decent horror movie, but if you aren't a fan of horror (and I'm not), I don't think this is going to change your mind about the genre.
IMDb link: http://imdb.com/title/tt0435625/