It's a very good film - good performances, good plot, fantastic direction that doesn't rely on it's genre stereotypes and just revert to gore and also less-defined character roles than the above posts would suggest.
It's definitely worth a watch. It's not necessarily enjoyable in the strictest sense of the word, but it is thought-provoking.
It is an excellent film. Brilliantly directed and written. When you think those two characters manage to carry the film for that amount of time, its nothing short of genius. It doesn't need Ridley Scott scenes with 5,000 extras or the likes. Just good, captive drama in its raw state.
I thought it was a bit gimmicky, too much of a one trick pony. With horror films often trying to prey on society's nightmares I guess it was inevitable that a paedophile horror film would come out sooner or later. Two character kidnapping films are pretty hard to write though, so disappointment is kinda to be expected. Not that bad a film, but certainly not one worth remembering 5 years after its release.
It's rather good tbh. I need to remember to buy it.