As somebody whose never read the book, I'm judging this movie PURELY on movie value.
As for keeping to the book, the only thing I know is Jodi Picolout's style - she tends to jump back and forth between characters, and the movie stuck to that. You get the thoughts of the lawyer, the mother, the siblings, and the father. In that aspect, I loved it.
As a movie itself, it was amazing. From start to end, the whole movie was a powerful experience. Not once did you believe the characters were fake. The relationships between the characters was near perfect. If you didn't know better, you'd say there were the real people themselves. The relationship between the two kids, sick with cancer, was a beautiful thing to watch unfold.
The soundtrack could've been better.
Notes to people seeing it: Bring tissues.