I lie between those who see this as a masterpiece, and those who see it as en empty exercise in clichéd sadism. On the one side, the film is very well made and effective. The cinematography, the acting, the editing, the score and direction work together to create a lot of truly creepy uncomfortable suspense, without resorting to too much gore, especially in the film's first half.
On the other hand, it lacks any thematic core (as opposed to, for example "Territories" a recent low budget horror film that also managed to raise questions about political prisoners and xenophobic paranoia). It also lacks any psychological insight although the first half hints that might be where the film is going. But these horrific intruders aren't ultimately reflections of the main characters inner lives, or a comment on their emotional struggles. They're just generic, unexplained evil. The film does a good job of evoking a nightmare, but by not aiming higher misses the chance to be something more special than an effective scare machine.