I thought it was a good adaptation of the book
I really enjoyed the book when I read it and was looking forward to seeing the movie. Overall I think it was a good adaption.
One big difference I didn't mind (though it threw me at first) was that they didn't keep the origin of hungry kids a mystery. Rather it was revealed pretty early on and then elaborated throughout the movie. It was probably a good choice to do it this was since it would have been to much of a info dump to go through it all right at the end.
Then the difference I didn't like:
-Changes to Miss Justineau's character. She seems much more insecure and nervous in the movie than the book. She's also much younger which really changes her from someone who had an adult life before the apocalypse happened. They didn't include her back story of having killed a child in a hit and run, which is an important character motivation for her in the book. Also because she's much younger we don't get the contrast between the generation grown before the outbreak (her and Parks) and those after (Melanie and Gallagher).
-No information about the wider story, such as the misery in Beacon. In the book we learn that it's the biggest settlement in the country and it's terrible. And we don't get the impression any other countries have it much better. This is important to Melanie's decision in releasing the spores so that "humanity 2.0" get a chance to start over. It would have been easy to include some tidbits about this through Gallagher talking to Melanie about his life(and thereby giving him some needed character moments).
Despite this I really enjoyed the movie. Most thanks to the stellar performance of Sennia Nanua. The story was all about Melanies journey and she carried it very well. There were many emotional moments but also thought provoking ones.
I've seem some complain about the zombie action scenes but I don't really care about them. The movie would most likely have been better with a little less action and a little more focus on the characters.
There is no such thing as too much time