No, I haven't read the book. But the up side of that is being able to answer the question of whether it's a good movie for those who don't have the book as background.
And the answer? Eh, it was okay.
Ender's Game is the story of Ender Wiggin, a young genius who gets tapped for Battle School and then Command School. See, 50 years ago the Earth was attacked by big, insect-like aliens called Formics (though in the book I understand they're simply known as Buggers), and ever since then there has been a focus on training young recruits to fight. But while people of Earth generally believe the training is a just-in-case measure, the truth is the military is headed for the Formic home world to end things once and for all. Kind of a "the best defense is a good offense" argument.
Anyway, Harrison Ford plays Colonel Graff, a man who is convinced Ender is the key to winning the war. And the movie is mostly about Ender's training, and his psychological development as he rises through the ranks, pushed along by Graff. It's probably not giving anything away to say [but SPOILER ALERT if you don't want to know!] that Ender does, in fact, succeed in winning against the Formics.
I get the feeling Ender's bonds with the fellow trainees who become his core crew are probably way more interesting and detailed in the book. But then again, books can afford to be more detailed; they don't have to keep to a short enough running time. Still, the whole thing felt like a kind of gloss in terms of action: tested, recruited, some fights, some training, some more fights and training, Ender wants to quit but they won't let him, he "graduates" and goes down in history. We see time and again how clever and innovative Ender is. How he thinks differently, how he's a good strategist. We see the ways in which he's able to get his crew to trust and respect him. But while the movie pretends to delve, it doesn't really. It's more an illusion of depth.
I will say Asa Butterfield did a remarkable job. He does so much more with his expressions than most actors do with their entire bodies. I hope he gets many more chances for solid roles; he shines in this one. Also Nonso Anozie as Sargent Dap—I can picture him as Gamby in St. Peter in Chains actually. Really liked him.
On the whole, not a bad movie but nothing amazing. Ender may be innovative, but Ender's Game is not.