Lots of people have read the book - and the movie assumes that you have. Or that you at least know the basic plot. If you don't, then go find a copy because it is so worth reading! (It's on my personal List of Greatest Books Written In All Of Our Lifetimes and Beyond - someday, I'm going to have to parse out that list). Spoilers from here on out -
Mostly, Ender's Game is about psychology and ethics. It's just set in space to make it more accessible and, frankly, more awesome. It's always been cool to me that it's a kid who teaches the moral and ethical lessons after the mass genocide he's unwittingly taken part of. The movie does not skimp on that at all. Just when you think it's all over and Ender's going to go home a hero - there's a little sequel hook that make you wonder if Speaker for the Dead will get a film (thought somehow, I doubt it. But stranger things have happened). I kept thinking of The Hunger Games, where you're just never going to get a 100% faithful film adaptation of the book, but it's good enough for the story they were trying to tell and the spirit of the story was kept intact (the whole Demosthenes subplot with Valentine and Peter didn't make it into the movie, which was a shame because I really liked that in the book - but it wouldn't have served the movie). I was a little worried that knowing the twist going into the movie would ruin it, but they kept that pretty well under-wraps until the Big Reveal. Even knowing what was coming, I totally felt faked-out just as much as Ender had been.
I could gush on and on about the story, but you can read that in the book. Strangely enough, for me, I've got to talk about the look of the movie. I think my favorite "setting" is the Battle Room. That has got to be the most gorgeous piece of CGI in any movie - anywhere. Not just the static room, but how the Battle School cadets move around and the overall look of the piece. The majority of the movie looks fantastic, but the Battle Room is the real eye-candy.
I also like the beginning of the movie - how the shots of Ender are composed so you're looking at him head-on when the story is establishing how he's a loner and a misfit (or, as Colonel Graff calls him later, a oddball). Those kinds of shot compositions aren't kept throughout the entire movie - just enough that you know that Ender is a bit of an outcast. Plus, the scenes set on Earth actually look like they could be a realistic version of the future. Things look like they do now, but just with a few futuristic details that create that kind of atmosphere. Also - the music is simply beautiful. I need that soundtrack in my life (if it doesn't at least get nominated for some awards... well, I won't be surprised because those award things are stupid and rigged. But it ought to be recognized for being awesome).
The acting was top-notch. Probably one difficulty in getting Ender's Game into production is the fact that, in the book, the Battle School cadets are, like six. Try finding a six-year-old actor with enough talent and seriousness to pull that part off. Even if you age the characters up in the screenplay - child actors aren't so great (Imma lookin' at you, The Last Airbender - among your many sins). But the kids in this were just phenomenal. Asa Butterfield is probably the best young actor on the planet right now, and with him sharing screentime with Hailee Steinfeld and Abigail Breslin, that's saying something. There were a few scenes where I just sat there and realized that there was this teenage boy acting against the likes of Harrison Ford and Ben Kingsley and actually holding his own. You believe that Ender has moved up the ranks in Battle and Commander Schools as someone that these battle-hardened officers can respect. Just - wow.
If you're a fan of Ender's Game and you've been waiting nearly 30 years for a good movie, you'll enjoy this. If you read Ender's Game because it was an assignment in high school (those of you lucky enough to have gone to high school before science fiction became a hiss and a byword in the realm of academic elitism), you'll enjoy the movie. Even if you've never read Ender's Game (in which case - Hi, Mom! Glad you're reading my blog!) - go read the book and then see the movie. You'll automatically become a better person for doing so.