I read Orson Scott Card's Ender's Game shortly after it first came onto the market some thirty years ago but I didn't rate it highly enough to keep on my shelves, so I haven't read it since. I could recall the broad outline of the story but remembered few details, so I watched the film with an open mind.
For any readers unfamiliar with the story, it is set in a future in which Earth is at war with Formics – aliens resembling giant ants – who had been beaten off after trying to invade some decades before but were now perceived as posing a renewed threat. The International Fleet defending Earth had discovered that children, intensively schooled in computer games, were faster at understanding and solving tactical situations in battle, so instituted a programme of training and selection to find the best. Their choice was Ender Wiggin, a boy who exhibited the right combination of intelligence, tactical control, and ruthlessness in battle. The story follows Ender through his training, climaxing in a final battle with the Formics.
I can't comment on similarities and differences compared with the book, as I read it too long ago. However, I formed the impression right at the start that the film was "the book on screen" type of adaptation, rather than a freer interpretation of the concept; the fact that the author was involved in the production might have had something to do with that. So the film starts with a rather clumsy voice-over infodump to explain the background to the story, about the aliens and the programme to train children, before the drama begins. It's the kind of thing that you might expect in a sequel, just to remind viewers what happened in Part 1. Once it gets going, the direction, acting and CGI are all handled competently enough, and the zero-gravity combat training scenes are convincing and entertaining. Despite this, I found that the film lacked a certain tension until the climactic battle; it had a rather routine, by-the-numbers, box-ticking air which left me feeling uninvolved. Good to have a conclusion which challenges the morality of an all-out interspecies war, though. It was just about worth watching, but rather forgettable, with the most memorable image being the Maori tattoo on the face of one of the characters!