Two and a half years ago, when I put together my Fill-in-the-Gaps list, this was one of the first books. For the sci-fi-fantasy community (on whose fringes I skulk) it's a staple; not to have read it is embarrassing. Plus, my dad, who has always greatly informed my reading list, read it a couple years ago and kept nagging me to get to it. I'm very glad to have read it, because I feel like it's become a cultural touchstone and now at least I can be part of the dialogue.
Premise: in a near-ish future Earth, the most promising of child-geniuses are sent to train in the Battle School, where they learn to fly fighter spacecraft in preparation for a coming war with the Buggers, an insectoid alien race who nearly destroyed the earth 80 years ago. Ender Wiggins, a six-year-old boy, has been identified by government agents as special--a genius with the capability to become commander of the fighting fleet when the war comes. He leaves his family forever (no contact at all until a family visit that's allowed once the kids turn 12) to face the rigorous, often merciless training at fight school, where he is stalked and monitored and presented with challenges the other students are not.
Themes: The book's biggest questions have to do with the nature of humanity (do the aliens have less right to life?), goodness (is Ender a bad person, because he's been hardened by his teachers into taking life opportunistically? is his brother Peter a "bad seed" type?), and education (is it right to design children through such rigorous training? what about if that's the only way to save the world?).
My personal reaction: like I said, I'm really glad I read this book, because not I can be part of the conversations that include it. I hear it referred to pretty frequently. I can't say I loved it, although I enjoyed the story and found myself caught up in it. Certain elements feel dated. For example, outside of the Bugger-Earth conflict, humans are divided in violent political factions that probably felt more plausible during the height of the Cold War than they do now. I also liked Card's writing of the battle training sequences and student interactions better than I liked the more allegorical and discursive parts of the story (long "telling" passages about his sister back home and her clandestine political campaigns, or the highly metaphorical and, in my opinion, not always interesting video games Ender plays to decompress). I also wish the story hadn't been so forcefully gendered. The reader only meets one female student in the Battle School, and she ends up cracking under pressure at one point. The narrative blithely explains that women have evolved differently and aren't as likely to be suited to the Battle Academy way of life. I find that to be another element of the story that seems falsely anachronistic (like the Iron Curtain feeling). But anyway.
Overall take-home: glad I can say I've read it. Didn't love it. Would like to talk more about it.