It is probably the best adaptation of difficult material they could have gotten, but they changed enough of the background story to completely change the meaning of the story.
In the book, humanity was fighting a desperate war for survival, in the movie we were pissed they attacked us once so we tracked them down and wiped them out.
A story that was supposed to be an examination of how far people will go to survive, asking what actions are justifiable to save your species. What rules and even mores are you willing to throw aside.
And instead we got another "aren't humans evil" and "look what the warmongering military does to children" movie.
I mostly enjoyed the movie because of the way they managed to get as much of Ender's struggle being shown as they did. They externalized as much of his thinking as they could through his discussions with Graff and that let the audience in on his thought processes. It was necessarily compacted to fit into a movie, but it worked.
If they had simply not felt the overwhelming need to make the humans the bad guys it would have actually been a very good adaptation.