Having seen the movie a bunch of times, I knew what to expect going into this book, but it still had enough difference that some pieces were still surprising. I think the book has a strong plot but doesn't invest enough in it's characters. They are all pretty flat and read a bit like archetypes stuck in the story. There isn't really growth or change, what you see is what you get. The most interesting part of the book is the setting and the survival attempts. Watching the group try to hold out through what seems inescapable is difficult. I think that alone makes it enjoyable and worth the read. This could have easily been a 5 star read with more character investment.

Main differences from movie (spoiler-free): extra characters in the book, additional detail to characters, additional body horror moments.