It's a thriller, it's scary, it's engaging. It's a monster movie, but the monster doesn't drive the story, the monster drives the scary but the story is driven by feelings of love, friendship, and a sense of duty on the part of the characters. That's a refreshing change from the typical monster movie type thriller.
It has some absurd little plot details but the story is done so well that you barely notice the woman running down the streets of Manhattan in heels for half the movie or the woman impaled through the shoulder by a 3 foot long piece of rebar sticking out of the concrete who just minutes after being lifted (yanked?) off the rebar is running down the streets (at least she's not the one in heels).
I'm not normally a fan of thriller movies, but I liked this one. Go see it.