It just came out here in the US, but it seems it came out in the UK about a year ago (and the UK's version is supposed to have a better ending). Anyway, this is one of the best horror movies I have seen in a while (probably one of the best period). They did an awesome job building up tension at the beginning, and the interactions between the characters as the events unfold are great.
So, if you like horror movies, go see it.