This movie was already spoiled for me because I read the book many years ago. But the movie can't help but spoil itself. It's a great film and one of the best adaptations of a novel to ever appear onscreen, but if you really know nothing about Ender's Game, and can read at a 9th grade level, honestly go read the book first. If you have time.
The problem is that by the time of Ender's "final exam," it's hard to imagine anyone in the audience sympathizing with Ender's shock that he hasn't really been playing a video game; he and his tween friends have been controlling actual spaceships killing actual aliens by the billions. The immersive, CGI photorealism of the game just looks too real; the audience can't help but accept it as real. The "simulation" in this film looks the same as diegetic reality in other Hollywood blockbusters! And we have been trained to suspend our disbelief! In other words, the photorealism of the battle simulations undermines the premise that they're not for real.
Commanding a battle from his combination IMAX and holodeck, surrounded by his subcommanders and fighter pilots, Ender can control a disembodied point-of-view at will. This POV has no physical or temporal limitations; it is seemingly omnipotent and all-seeing. This suggests that within the diegetic universe, the images really were simulated from live data feeds, unless the hawkish grown-ups have a magical flying camera transmitting video by ansible. But to the naked eye, could anyone, real or fictitious, distinguish between this graphic simulation and a live video feed? To the genius Ender, who's used to playing with Shrek-like graphics on his iPad, doesn't the life-likeness raise suspicion?
Orson Scott Card, who wrote Ender's Game in 1985, was also a producer on this film. The film stays true to to the book without ever feeling burdened: it manages to recreate the key episodes and characters and tie them together in a way that evokes the emotion and meaning of the original novel. The only drawback here is that things have to happen a little too fast to fit within two hours. In the book, it comes as a shock that midway through his supposed education, he has already won the war! In the movie, we know it's time for a climax.
One highlight of this film is the training room, a weightless 3-d solarium with re-arrangeable blocks. The students play a version of capture the flag with paralytic suits and light guns, thinking in three dimensions, as they would in a space battle, to beat the other team. It would have been great to see more of these scenes and some of the ingenious tactics dreamed up by Card in the novel. As there was recently in Gravity, there are some well-timed push-offs and counter-rotations to get our protagonist sailing toward the right aperture. And there is one formation that Ender later uses to win the war.
P.S. If you're wondering why I didn't spoil Gravity, it was just too fuzzy and wholesome. But if you want to see every space spation, shuttle and valuable piece of technology in orbit get shredded by debris travelling at 20,000 miles per hour, you should see Gravity. Also if you believe in Murphy's Law you should see Gravity.
P.P.S. The moral of Ender's Game is that ants are people too, so think about that the next time you reach for a can of poison.