I saw the movie before the book, so I was curious to read the book afterwards, even if I wouldn't give the movie five stars. But I read a lot of reviews that said the movie differed dramatically from the book, so I decided to find out for myself.
The book was a bit of a struggle for me: I found Ethan and Lena's relationship to be shallow, unfilled, and Ethan was just plain unlikable. But, there were aspects of their relationship in the book that were totally absent from the movie. For example, Lena and Ethan couldn't telepathically communicate.
The book does bring a lot more mystery than the movie: the curse Lena is affected by is not brought up until much later in the book in comparison to the movie. Still, I was okay with this. I would have enjoyed a bit more mystery, but in the adaptation I didn't have a huge problem with this change.
The ending also had a few big changes from book to movie: Ethan's father never appears in the movie, which leads to a few big changes, and Ethan never looses his memory in the book. These two changes, among other things, make the ending of the movie and the potential for a sequel quite different.
Overall, I think the movie was a bit more entertaining than the book, but I didn't love either! They're alright, if you're bored and into supernatural love stories!