I would say that the movie effectively caught the spirit of the book. The movie faithfully shows that Freak and Max are two very close friends that would be willing to do just about anything for each other. The movie is similar in that Freak has some kind of a disease as well.
Unfortunately, anytime a movie is turned into a book, some things are cut or added for cinematic effect. It's the main reason why I don't like book to movie projects. The movie adds a basketball scene that was never in the book for example. Freak never sat on top of Max's shoulders to win in a pick up game of basketball.
One example of something the movie kept from the book but completely changed is the death scene. In the book, Freak had a seizure at the Christmas party. He was taken to the ICU, and Max and Freak were allowed to talk. Freak talked about what his new body would be like after surgery. He meant his new body in heaven, because Freak died in the hospital. That death sequence is completely different in the movie. Freak has a stroke and dies at home. Gram tells Max, and he immediately runs out to find Freak. All he is able to see is the ambulance leaving with Freak inside.