I had a rare opportunity to watch a movie on its opening weekend. This adaptation of one of the best known science fiction novels/series had its successes and misses. I'm deliberately not saying "failures" because I think many were understandable. I'll start with some differences from the book and then mention some other spoilers further down.
If I was going to question something in the book it would be regarding the battle (in the spoilers below).
Overall, the movie did what was intended - it entertained. However, I don't see a sequel as a huge probability.
The book had a more detailed side-story where Ender's siblings played a key role by creating fictional characters who debated the war. This set them up for gaining power back on Earth, which was Peter's goal all along. While it was interesting and led to Orson Scott Card branching off the Ender brand through them, including that could have overcomplicated the plot for this movie. It would have needed at least another hour.
This may be a bit of a spoiler but everybody knows there was an epic space battle in this movie. In the novel, when Ender was working through various final battle simulations it was interesting how in come battles, the human ships were decades old while in others, they were the latest technology. They didn't go into an explanation of the ships in the movie but they all looked same era.
Although it didn't have a lot of coverage, I liked the use of the game in the book.
As I recall from the book, there was a bit more character development with Bean also. He had more strategic importance than Petra, but I guess for a movie it was better to develop the female character. But in the book series, this story was retold from Bean's POV in Ender's Shadow. Petra's role in the movie battles was very different from what I remember in the book - see the spoilers for more on that.
One other interesting component for the novel was that Ender was groomed to be a hero, he was going to be a hero everyone feared. When the battle was over and humanity no longer had a common enemy, factions would fight for all of the battle school children, but none more than Ender.
First. My reference to the varying ages of the ships. That was a clever way to show how humanity has actually been sending waves of ships to the alien's outposts and home world for years. By going near the speed of light, months seemed to pass for crews when years or decades passed.
Another clever, and subtle component was when Harrison Ford turns to the display that gives a warning of an approaching invasion fleet. They don't go back to it, but I thought it was odd (knowing that there was never an enemy fleet approaching). Then in the battle scene I realized that it was estimated time for the human fleet to reach the enemy world and fleets.
Bean and Petra. The movie didn't really do enough to develop Bean. In the books, he was being groomed to potentially take over if Ender couldn't finish. Bean was important enough to have a spinoff of the series.
Petra seemed to be developed in the movie as Ender's right hand person, but when they went to the actual battle, she ran one ship. Didn't it have a crew? She had enough importance to be transferred to Dragon Army and be part of Ender's final team, but then she just ran one ship that only had one chance to fight or shoot? It seems like the had her on the team just because she was Ender's girl friend. Seems a bit sexist and condescending to the character. Oh well. It was entertaining. I'm glad Ender found the queen at the end even though I'm not itching for a sequel.