I was just coming here to start a thread about this film, glad there is one already! :D
I just finished watching this, it's freaking AMAZING. It's classified as a horror movie, but I think that's a bit misleading.....while it is certainly horrific, it's not in the same vein as something like your typical slasher/ghost flick. It's more a psychological study of humans IMO...I kind of look at it as being in the same kind of vein as Dancer In The Dark or Funny Games. It's really bleak and difficult to watch, but at the same time totally engrossing. The cast and crew do a brilliant job of portraying real despair and torture without really being all that graphic about it. There is some gore, but I think the actually tormenting stuff comes afterwards, and is down to very masterful direction, acting and make-up rather than splatters of blood.
Awesome casting, awesome work put in from the two lead girls (and everyone else), awesome film all round. It will definitely exercise your brain.
EDIT: probably worth mentioning that there is some depiction of self-injuring in this film, so be aware of triggering if you need to be....it occurred to me that I don't think I would have been able to watch some of the cutting scenes in this movie 5 or 6 years ago for that reason...