As usual, the movie is not as good as the book, which is to be expected. Yet, even if one hasn't read the book, this movie fails at being a movie in such a big way that it no longer matters whether you have read it or not.
There is one major way that this movie fails, even if you leave out all of the major plot points and moments that it ignores from the books. It has absolutely no character development. This is in part due to how short it is and how much story is being crammed in, but that is no excuse. I think it was a couple two minute long walking scenes where Todd transitions from not liking the girl, to caring about her deeply and that is not how you tell a story. Every single character is static throughout the entire movie.
And if you have read the book, I really don't think I can list the amount of ways that this movie falls short. One example I can think of that has upset the most people is Manchee. In the book, Viola is kidnapped by Aaron and Todd tracks them down. In a huge character development moment, Todd has this sweet moment with Manchee where he acknowledges how proud he is of him even though he has been a tsundere the entire movie and then they both get to rescuing Viola. In the long and short of it, Manchee dies protecting Todd. It still sucks, but it is meaningful.
I've seen one star reviews based solely on the dog being killed. Those people are idiots, but the writers should have still seen this backlash coming. If they could skip out on all the other meaningful scenes in the books, they should have been able to hold back and not give Manchee a completely meaningless death. There are many many many other examples, but then I would have to write a book and I don't want to do that.
Basically, if you read the book, you'll understand why this movie is complete and utter garbage.