Great movie. This is an apocalyptic thriller, the same genre as the Cloverdale movies. The cast was awesome: John Malkovich down to MGK...very cool.
There were some unanswered questions, but some mystery was important in order to maintain the tension throughout the movie. This was not a sci-fi and not a horror movie in my book. It was more of a psycho drama, using a fantasy monster(s) in an otherwise normal present day Earth.
The ending made a lot of the drama and tension worthwhile, giving a kernel of hope from an apparently hopeless Apocalypse.