Full spoilers ahead
I'm not normally one to compare books to movie. Movies are a completely different way to tell the same story as a book, and I usually enjoy movies more. There are things books can do, like detail what's going through a person's mind during any given situation, that a movie can't replicate. Likewise, movies can entertain in ways books rarely can.
The problem movies run into is when directors change the story or ignore big chunks of the story in the book. The latter is what happened with "Ender's Game."
Don't get me wrong, the movie was good. The story of Ender Wiggin was told accurately, albeit a little too brief at times (seriously, the pacing of both the book and the movie were awful. Transitions were way too quick and unclear). Ender (played well by Asa Butterfield) is a prodigy child who has been chosen to lead the human army against an alien race that attacked Earth 50 years ago. Legendary Fleet Commander Mazer Rackham was the hero who saved Earth the first time around, and now Ender is believed to be the next savior.
Throughout his training at the Battle School space station, Ender quickly ascends the rankings and makes new friends (and a few enemies) who join his arm. He is a tactical genius and learns how to handle himself as a leader.
This was the first misstep the movie made. The battle room was the coolest, most entertaining part of the book, but the movie had only a couple quick scenes, really only one extended scene. This was a key aspect of Ender's development as the prodigy he was, but the movie simply brushed through it. Very disappointing, especially considering the one scene they did show was cool. The battle room is an enclosed arena where two teams shoot each other down (the combatants' suits freeze when struck, rendering them useless for the remainder of the battle) while trying to get to each other's base, all while in zero-gravity.
The competition and the need to win became a big driving force in Ender's development. He was so good, in fact, that the commanders would send two teams against Ender, and they would make Ender's team fight multiple times per day, both of which were unheard of. Yet he still stayed atop the leaderboard. Ender's endurance and test of mettle in the movie was far hollower.
In some ways, I like to think of "Ender's Game" as a futuristic space version of "Hunger Games." Kids were being used in morally questionable ways. This was the crux of the story that young director Gavin Hood (X-Men Origins: Wolverine) focused on in the movie and absolutely got right. Colonel Graff (played by Harrison Ford) and Major Gwen Anderson (played by Viola Davis) are the Command school big wigs. They are monitoring Ender, trying to understand what motivates his every move.
It started with a simple fistfight. Butterfield exuded confidence while calculating his moves, similar to, but not nearly as good as, Robert Downy Jr., did with his Sherlock Holmes character. Ender showed the pain he felt while beating his adversary mercilessly, which was supposed to send a clear message not to engage in a fight with him anymore.
That's what Graff wanted in a commander. A leader who wouldn't be afraid to eliminate enemies at all costs and eliminate future battles. Graff nailed the "It's us or them" mantra into Ender's head during simulations.
Where this becomes morally questionable is when Ender isn't given all the facts. Leading simulated battles is one thing, but in real combat, would Ender lead the same way and make the same sacrifice along the way? When Ender finds out the simulated battles he leads are actual, real-life battles, he is devastated. This is when Ford and Butterfield deliver the best dialogue exchange of the movie. Butterfield shows a range of emotions—betrayal, anger, remorse—exactly as the book did.
There were two major aspects of the book that the movie ignored. One involved Ender's siblings, Peter and Valentine, who created alter egos using pseudonyms and a worldwide media connection to influence entire countries. This story was interesting, but not pertinent to the rest of the story, so its exclusion from the movie was not a big deal.
The understanding of the alien race, which directly affected a big reveal during the ending, however, was a big deal. The aliens found ways to communicate to Ender (mainly through this cool computer game, my favorite part of the book). After Ender destroys the alien's home world in one final, epic battle (done very well in the movie), one last queen alien reaches out to him and reveals to him that they never meant the human race any harm.
This completely changed the psychology of the book, but the movie didn't touch on this. This realization led Ender to sympathize with the aliens so much that he did what he could to preserve the species and let them start anew.
This is definitely a movie worth watching, but to me, it's one of those less-common instances where I'd recommend reading the book first.