A multi-faceted story set in some future of humanity: on one hand it is about highly gifted children and how this separates them from the rest: Ender is not only special compared to the average children, but even in the selected group of children to go to International Fleet training. On the other hand, it is about a war between humans and alien 'buggers' who attacked humanity in two invasions some hundred years ago. And the children are a selection to train for being soldiers and leaders in this war.
Yet, the most fascinating aspect to me is the description of the trade-off between happiness of the individual (here especially Ender's) and the pressure exerted on him to learn to build and use his potential to the fullest. In that respect, it is also strongly connected to the question how much we are or even can be in control of our life.
It is also about the trade-off needed between the empathy needed to understand the enemy and the problem still to be able to fight and kill.
Maybe a bit melodramatic, but somehow this story really sucked me into it and kept me reading.
And the end of the book has yet another fascinating turn opening up the room for continuation.