"In the moment when I truly understand my enemy, understand him well enough to defeat him, then in that very moment I also love him."

Ender's Game is a movie with many flaws, but many qualities as well. Keep in mind that it is hard to just categorize it as a good or bad movie because of that.

No real spoilers ahead.

Story:
Like someone pointed out here before me, the reason why kids are necessary and why that's humanities only hope is left completely unexplained. The lack of other commanders, besides the one in training school, is pretty odd to say the least as well. In 50 years not 1 trainee from the academy passed the test. So what happened to all those failed kids? Especially the ones that did get to the final test. They know a secret that cannot be known to society (about the hero commander.)
There was no real character development in this movie at all except for the main character, and his development was very thin. The 2 friends he made do not have much in common with him, nor do they have any reason to like him. Especially the girl, since she is in a winning team for a while. After Ender's 'problem' with the Salamander leader, he feels bad about it. But as soon this part is over he never seems to look back at it or takes any lesson from it.
On the brightside, the massive plottwist in the end was surprising. After watching a movie with a rather unsurprising (but not boring) story development, I did not expect that. They could've singled out the emotional/psychological aspect after this part a bit more to make up for some lacking character development earlier.

Visuals:
Special Effects were lovely. A real sci-fi feel with great spacey environments that didn't feel unrealistic or cheap. The funniest thing was how they could've had computer games with about the same graphics we have now :P
I really loved how they did the battle scenes between plains and fighters, both in air as in space. You really got sucked into the battle field, and lost the feeling you were watching at a screen like happens so often with big battle scenes with lots of SFX.

Music:
This has to be adressed. I think this was probably the best part about this movie. Steve Jablonksy was the one who composed it. He also composed the music for the game Command & Conquer 3: Tiberium Wars, where he surprisingly managed to not dissapoint after stopping Frank Klepacki's monopoly for the franchise. And that's while Klepacki set the stakes very high for him. And in mainstream modern movies he is probably best known for doing some music for the Transformer movies, too bad those movies suck too much.
He did an amazing job on the score for Ender's Game. Bombastic, classical and majestic music that made every scene way bigger and impressive. Something you could easily listen to and enjoy without looking to the film, but just play really loud on your speakers.

Acting:
This is always something that falls a bit short when kids are the main characters in a movie. Remember the golden rule: Never work with children or pets when you are making a movie.
Not that it was bad, I was actually positively surprised with the acting job most of the younger characters delivered. The worst acting came from the adults xD But even so, the acting was not something award-worthy. Just don't expect big personal acting extravaganzas and you will be just fine.

Enjoyment/Overall:
I really had a lot of fun watching this movie, therefore the end(er) ordeal from me falls into the category good movie. It has enough speed, nice visuals, great music and it is just fun to watch.