Some of the themes within the book are left behind. That is to be expected during any book to movie conversion, but the themes chosen also change the way the movie will appeal to certain audiences. Ender's Game the movie is a much more cerebral story than it's sibling novel. It focuses on the choices of governments and the path to victory while diminishing the tales of love of leader and tactical genius. In all it is a solid adaptation, pleasant and sometimes ponderous it is not sci-fi for the sake of sparkles.
Viola Davis as Major Anderson may be the performance that most points out the changes. This is not because she is female, but because she is the moral barometer. She sees the damage being done to the children, not just Bonzo, but all of the children. As their youth is sucked up and mental wounds leave scars that guide Ender further down the path for which he is molded Anderson is powerless to stop Major Graff (Harrison Ford). While this story exists in the book it is not nearly as powerful. Within the movie it is part of the guide as to what you should take away besides the showy Battle Room and simulator scenes.
And those scenes are quite showy. They sparkle with the best that science-fiction has to offer but without dismemberment or gore. There needed to be more of these scenes and more references to how the various armies are performing in the standings. Because the scenes are so short and there are few references to performance in the Battle Room during this act a viewer has no idea how many games Ender commands, nor what he does that makes him qualified to advance.
What the movie gets very right is the relationship between Bonzo and Wiggin. You see the hate and pending violence from the first moment. Their confrontation in the bathroom is brutal. My wife was shocked (she has not read the book) at the savagery displayed by hero and victim alike.
In all I would recommend this movie to people who've read the book as it stays true to most of the main themes of the book while cutting some corners. For those that have not read the novel this is a different kind of sci-fi pic. It is much more thoughtful than most of the blockbusters out there. The thoughts you have after seeing it will be a bit disturbing and a bit inspiring.