I saw the movie first. I was surprised that the movie was really close to the book. On the other hand: the book is so short there wasn't much room to leave out anything. That's my only critique really: way too short. I finished the book in one sitting. I enjoyed that sitting: the characters are believable, no weird motivational jumps, no unexplained skills or out of character actions. The emotions of the characters are credible. I was put off at first by the first person narrative but it turned out fine. If I had not seen the movie it would probably have been full of suspense. Even so I could not put the book away. Only four stars because all in all it is a bit too simple for my taste: the writing style, the plot, the characters - it all screams "young adult" and the books I read right before were the ASoIaF series ... a stark contrast.

So: if you enjoyed the movie it is worth a read and if you haven't seen the movie it is still worth a read.