I was just trying something new here. Horror isn't really my genre, but a friend really liked this book so I gave it a try. I like aspects of it... the idea of never really being sure what the nature of the monster actually is. I can imagine that if Earth were invaded by aliens, or if monsters were created in some government experiment gone wrong, or if some portal opened up between Earth and (??somewhere??)... we wouldn't necessarily know the whole backstory of the threat, and how it came to be, and what might be done about it... before society completely collapsed, leaving the survivors scratching their heads wondering what the hell just happened.
The book captures the claustrophobic mood of people trapped together in close quarters very well. And there is a particular plot point I don't want to spoil, but I am reminded of Nightmare on Elm Street, where the victims can never let themselves fall asleep. Also, something about this has a vibe of Day of the Triffids, but I can't quite put my finger on it.