This was about as good a film adaptation as I could expect.
Given how there's no way any of the other books get adapted (at least not in the Ender branch of the series), leaving out the Locke and Demosthenes subplot was perfectly understandable. It is also understandable because it would have been incredibly boring on screen.
I think the only thing missing was more conflict between Ender and Graff. There was no sense during the time at Battle School that Graff was pushing Ender to his breaking point, which made the climax of the film a little less powerful.
Other than that, I really enjoyed the film. Thought the time at Battle School was really well done overall, and the Battle Room itself was amazing in IMAX. Harrison Ford wildly exceeded expectations.
I was a bit disappointed with some of the deviations from the book. Some were understandable and necessary; but there were a few that just took away from the movie. One in particular really stuck out for me.
Spoiler Below: Highlight text to read
Bean: His arc in this is altered to the point where he is not a necessary part of the story. It feels like he was kept in just to utter his big line in the final battle; but the build up to the line made no sense. Had the scene for the final battle room game played out like it did in the book "the enemies gate is down." would have had the same desperate significance that it had in the book.
As a whole I enjoyed the movie, once i was able to step back and watch it from the perspective of someone who hasn't read the book it became much more enjoyable. Asa Butterfield and Harrison Ford are both excellent as Ender and Graff. It's just a shame that they didn't share more screen time together.
If you loved this, then you have to check out their BBC show that came before it Spaced. It's airing Mondays on Trio in America starting this past Monday. Probably my favorite television show of all time.