I love Roal Dahl to death and I will always consider him one of the best authors for children's book (and Quentin Blake, one of the best illustrators; they are darn funny).
Having said that, I have to admit that this may be my least favorite book from him. First of all, I missed that crazy character who always talks in exclamations points -not only Willy Wonka talks like this, but also the bugs in "James and the Giant Peach"; the Grandma in "The Witches"-.
Secondly, I did not like Mrs Trunchbull, and not because she is the Evil in the story, but because of how violent she was, especially to children! And not even one of the teachers interferes. Maybe I am reading to much in this, but I cannot laugh when a big bully (a headmistress, nonetheless) throws children out of the window, or locks them in a tiny room with hundreds of tiny glasses in all 4 walls so they can't sit down, or makes a kid eat a whole chocolate cake in front of the entire school and all the other teachers are.. where?
The movie is very loyal to the book. And Matilda is a sweetheart.