Ender's Game: It was okay, I guess. Not good, not bad. It worked though. I was entertained at least! It does stand alone as a movie, but it definitely feels rushed, even apart from the source material. Things move at such a pace that you lack the time to build up empathy with any of the characters, and it also results in very iffy cohesion in many parts. Once they get to Command School, things are kinda alright, but the very end was... not the wrong choice, but poorly done. Together with the accelerated timeline (it's shown that the entire movie once they get to Battle School happens inside a month), it opens up a plot hole a mile wide that the bugs would have to be precognizant in order to install that on the planet, instead of it being done in the weeks/months/years Ender was playing the Mind Game/fighting them.
As an adaptation, most of the streamlining done definitely works. They chose most of the right parts to cut (Locke/Demosthenes), the characters are condensed more or less as they should have been (sans Bonzo for some reason being in Rose the Nose's body).
On the negative, they really suffered from a Tell, Don't Show in terms of Ender's empathy with the enemy. It's harped on constantly but they really don't do a good job of SHOWING it ever, making the last scene even sillier than it already was. Also, the use of certain verbatim lines from the book entirely takes you out of the movie sometimes from how awkward they end up sounding. They did a good job mostly with the isolation aspect, but it should have been played up more. He recovered too fast, too easily, like an afterschool special. If I could add one scene, it'd possibly be playing out the whole part where Dap yells at the rest of the Launchies for not opening their lockers yet, just to twist the knife and underline the staff's attempt to isolate him. Other than that, maybe even one line with Ender explaining to Petra why Bonzo beating him up on him for saving his ass was stupid, just to show the audience his understanding of command. Small bits and pieces, but they would have helped. Gotta rush through. LotR can be 3 hours, but this had to fit under 2 for some reason...
On the nitpicky side, I think the Salamander ambush fight and the two army fight should have been split into two separate scenes (as a part of extending the movie). It was done well enough, but doing them both and showing them on the same day like they are in the book would have played up the unfairness and exhaustion factors. Also, the fact they Million Dollar Baby'd Bonzo was hilarious, as was Bernard ending up in Ender's jeesh. On the one hand, it shows Ender's such a good commander that he can make use of even a terrible soldier like Bernard, also saving the introduction of another character! On the other, it's... just kinda hilarious knowing the source material, which is why this is in the nitpicky section rather than Negative.
If nothing else though, it made me believe an Ender's Game movie was genuinely possible. There are some actual problems with the movie, but most of my complaints are nitpicky at best. They definitely hit the board, but they missed the bullseye.