Executive summary review: In my opinion, they did not screw it up. If that's all you were looking for, go ahead and go see it and let me know if you agree.
For those who are now confused, Ender's Game is based on a book (which in turn was based on a short story). The book won the Nebula and the Hugo awards, has been suggested reading for the Marine Corps, and was extremely influential in my youth. It's kind of a big deal.
Before I get into the rest of the review, a word about Orson Scott Card. I'm well aware that Card's political and social views are controversial at best. Much worse can be said about them. I do not condone his positions, and will not defend him. If you object to seeing this movie because you dislike Card's opinions and actions, I will not try to argue you out of it.
One of the worries that has often been expressed about a movie adaptation of Ender's Game is that the main character is mostly nine or ten years old in the book. Asa Butterfield, who plays Ender in the movie, is 16 as of this writing, and was probably 15 for the majority of filming. Clearly, this is a very significant difference, especially because Ender's youth, and the nature of childhood, is a central focus of the book. While some of the impact is indeed lost because of Asa's age, he does an extraordinarily good job of expressing vulnerability even while his character is strong, and innocence even while he is the smartest person in the room. This is the fine acting performance that I was hoping for when I heard he was cast as Ender, based on his work in Hugo.
The rest of the children are mostly solid, but with few standouts. There are a lot of them, and very little time and screen to establish personalities. One of my minor gripes with this film is that Petra was not as strong a character as I had hoped she would be. She has a prominent part, as she should, but played more as a mentor/almost romantic interest than as a person in her own right. This is especially disappointing because the actress playing her, Hailee Steinfeld, was brilliant in True Grit.
This is the most ethnically diverse cast I recall seeing in a major Hollywood movie. The driving force of the plot is the International Fleet, and I am very glad that the studio actually used the right actors for the jobs. There is the matter of casting Sir Ben Kingsley, who is Indian and British, as the half-Maori Mazer Rackham, but he does a fine job in the role. His tattoos, while not mentioned in the book, serve a purpose in the narrative of the movie.
Women don't fare quite so well in the movie. While there are a number of girls in the background, there are only four female characters to speak of, and only one of those is actually her own character. Petra, Valentine, and Mrs Wiggin all exist to support Ender, and have no narrative role other than this. Only Major Anderson is really a strong character, though she is quite good. Interestingly, her role is the most substantially changed from the book, in which Major Anderson was male, and was the officer in charge of the game, not a psychologist.
There are, of course, other changes from the book. A number of plot points were rearranged or compressed in order to fit into the movie format. There are two changes that I consider particularly important, though. One is the complete removal of the Locke/Demosthenes plotline. I understand that this needed to be done for time, but it's still disappointing, as its absence removes much of the context of what is going on in space. The second is the removal of Command School to a place a bit farther than the one in the book, and the apparent introduction of faster-than-light travel. While the movie can (and does) gloss over this point, it's a pretty significant change to the setting.
The visual effects in the movie are good, but not awe-inspiring in the way that Gravity was. The Battle Room sequences, which are still crucial to the plot even though they're compressed heavily, are quite good. It looks like the weightlessness simulation was done with wires, but well done. Since actual microgravity is hard to come by for filming purposes, and the kind of tricks they used on Gravity couldn't work on this scale, I think they did the best they could.
Ender's game is a story about youth, intelligence, humanity, and violence. The film is not as moving as the book, and misses out on a good deal of the political commentary, but is still powerful.
2. Much Ado About Nothing
3. Now You See Me
4. The World's End
5. Ender's Game
6. Despicable Me 2
7. Star Trek: Into Darkness
9. Pacific Rim
11. Iron Man 3
12. Kick Ass 2
13. Man of Steel
14. Jack the Giant Slayer
15. Beautiful Creatures
16. The Mortal Instruments: City of Bones
17. The Family
19. Oz the Great and Powerful
21. G.I. Joe: Retaliation
22. The Wolverine
24. Monsters University
25. Hansel and Gretel, Witch Hunters
26. The Grandmaster
27. Machete Kills
28. The Great Gatsby
29. The Lone Ranger
30. This is the End