In my perspective, it's all about the actors chosen, and their performance. And I must say they didn't disappoint! In fact I think they all do an excellent job turning what could have been a disastrous endeavor into one very well done! I've never heard of any of the actors before but they definitely rocked it!
In general I think this movie is a great essay on evilness and Human's perversity. It's really sadistic, from the psychological point of view, and worst of all, it offers no chance to the victims to escape, from the beginning!
It's really a cat-mouse game where we know who will win... Evil! Since the beginning we see that evilness will triumph over goodness in this film. There's no chance to escape from it.Because evil also triumphs too many times in "real life", and the "heroes" that we're always watching in movies don't exist in a great number out there.
This film will keep you frightened and glued to your seat. Because one thing that this film is NOT, is predictable. It also contains a surprise scene that temporarily shifts into the realm of fantasy, which further bends minds and entertains the audience.