Firstly, I don't think most of the criticism claiming that the movie's plot is unoriginal is fair. It's a basic setup--one that has been utilized many times and long before Them. Rather than being trite, I feel like it's a staple of the home invasion subgenre.
Technically, this film is sound. Great cinematography, sound design, and acting. The direction is above average, with the exception of some of the shot design. I genuinely cared and worried for the characters, whether it be their unstable relationship or surviving the night. It is because we are so entrenched in their lives that the emergence of the “Strangers” becomes so unsettling.
Creating some of the most tense and creepy moments I have seen out of any recent horror film, there are scenes that had the audience literally calling out and squirming with anxiety. When the film is going full throttle, it really works and has you hooked.
The biggest downfall to this film is pacing and storytelling. The middle, as many have said, drags. This is for two reasons: repetition of scares and lack of direction. The same scare is recycled over and over (where we see the killer slowly revealed behind the unknowing victim, and then when we return to the same shot, the masked assailant is mysteriously gone). This works very well at first, but then gets tired. The other issue with the second act and early third is that Liv Tyler has little direction for her actions. She meanders through the terrain with little purpose--or at least that purpose was cut from the film, as the film often skips beats or jumps ahead in the narrative without clueing the audience in on the intermediate steps.
Of course, the ending is a bit jumbled and carries on far too long. Perhaps a studio's tinkering job?