First, the movie–insomuch as it follows the book–follows the book closely. There is a lot left out, but little if anything is added or changed.
Second, the movie–as far as what it does leave out–leaves out a lot. With regards to what the movie intended to cover, what it left out is not too much or bad. But, for those who love the book and everything that is going on, it will feel different and you will miss what it is lacking.
Third, the movie does change, or at least emphasize, one thing a bit more explicitly than I remember the book doing. The movie seems to take a stronger stance, a more explicit stance, to the immorality of what the military does with Ender and the military enemies. Readers will argue that it is there, but the movie makes it plainer.
Fourth, the movie suffers from something I think all movies suffer from as a result of the medium. There is a lot going on mentally and emotionally with Ender in the book that cannot be made as clear or as plain in the movie. It can’t tell you every thought or feeling he experiences without distracting from the film the way the book can. That’s sad because his mental and emotional anguish is a big part of the book.
Finally, let me say the movie is entertaining and worth watching. It can provoke many of the same types of conversations that reading the book can, if only to a lesser degree or with a bit less detail and information. I’d recommend it to others.