I've seen a lot of complaints that this movie didn't cover every scene, plot twist, character development, etc. in the book. It seems like many reviewers still don't understand just how different novels and movies are as creative media. For movies, time is precious - most movies have only around 2 hours to tell their story - whereas books generally don't face a hard limit on the page count. Moreover, movie are expensive to make whereas books are cheap (basically the author's living expenses and and market campaign).
Looking at Ender's Game as a movie and not just as an adaptation of the book, it actually holds up very well. Asa Butterfield manages to subtly portray both a ruthless and compassionate side to Ender. The rest of the actors generally do a good job bringing their characters to life, which is no small accomplishment give that most of them are kids. Even though the kids are older than their counterparts in the book, they still look, feel, and act like kids. Ender's voice even cracks a bit, like a kid going through puberty. While Ender and Petra do have quite a few scenes together, I was very pleased to see that Hollywood didn't try to insert romance into their relationship.
Gavin Hood, the director, obviously realized that he wasn't filming the book and the script makes some significant but smart changes. The arc of Ender's siblings - Peter and Valentine - is drastically curtailed (even in the book, it was only peripheral to the main plot). More importantly, Ender's character arc isn't quite as exhaustive or exhausted as in the book. I appreciate that Hood implicitly acknowledges that movie Ender hasn't been at Battle School nearly as long as book Ender and the movie doesn't try to cheat and shortcut that experience. Ender's arc feels appropriate for what we see in the movie and going further probably wouldn't have been possible in 2 hours.
Without spoiling anything, I was also impressed that they spent at least some time on the denouement. Not since Lord of the Rings: Return of the King have I felt so glad that a movie took time to give its characters at least some room to experience the fallout of events. Again, it's not the book, but the the essentials of the store are there.
That said, I kept finding myself frustrated that the movie wasn't longer than 2 hours. There were parts that felt rushed or condensed. The beginning in particular has an odd way of introducing Ender's universe to audiences, most of who are not familiar with the book. The first 10-15 minutes are probably more confusing as the movie really throws you into the story pretty quickly. I thought overall the movie was better paced as it progressed, but even later scenes could have used more time to simmer. If anything, I thought the movie included too much from the book and tried to cover too much ground at the risk of not developing the main plot sufficiently. Some important characters, such as Mazer Rackham, just don't leave much of an impression because we don't spend enough time with them.
I almost got the sense that Gavin Hood was forced by the studio to do some last minute editing to shorten the film. I would have thought that Good could have convinced the studio to allow a longer movie given the popularity of the novel, but that extra half hour probably would mean at least one less showing per day in most cinemas. I can only hope we'll see an extended version on DVD.
Overall, this is a well done, if condensed, version of the Ender's Game novel. It's probably much more faithful to the book than most movie adaptations of novels (certainly more so than say the Hobbit). If you go into the movie with an appreciation for the realities of filmmaking, I suspect most fans of the book will enjoy seeing their favorite characters come to life.