One of the first books I read on purpose was Ender's Game. I knew it would be a movie someday based on how awesome it is, but I never expected it to be a good movie. The problem is that there was just too much happening for one movie. Having seen the movie, I can say I was wrong. A lot was lost from what they had to cut, but they kept enough to make a good movie. The following will be my comparison of the two, with lots of spoilers.
I expected Bean's roll to be expanded since there are books that follower this character. I didn't think this was a good idea, and was happily surprised that they didn't.
Harrison Ford's character, Colonel Graff, was extended, which I expected and enjoyed.
I was surprised by the expansion of Major Anderson, who was well acted by Viola Davis, who I'd never heard of before.
Peter and Valentine were drastically cut. This is my biggest disappointment. They had to make cut to make it one movie, I understand that. In the book Peter and Valentine go about taking over the world in Ender's absence, and make good headway. This subplot that comes together at the end, really making the story for me. They did make a good movie without it, but they could have left it to make two great movies.
On the bright side, I was glad to find they fixed the part that happens after that. The ending of the book was sort of shoehorned into the next book in the series. They were two unrelated stories before this. The result was a little silly, and brisk. Instead of sending Ender off to an alien world where the aliens had reshaped the landscape for him like in the book, the movie actually fixed this by making one of the human bases be a former alien world.
What a brilliant change. It's simple and it works. Then they went further. Instead of the aliens reshaping the landscape to match the game, they communicated the existing landscape to him through the game. Again, simple, and elegant. I couldn't have fixed that any better myself. The result is that emotionally, the story ends in a logical place. It ends the story at the end of the movie.
I highly recommend the movie, but I still recommend the book highly-er.