The book is a cornerstone i classic sci-fi, and the movie is not far behind.
I recently read the book and I was quite excited to see how they would make it into a movie. There are so many pitfalls for movie interpretations of books and for sci-fi and fantasy in particular, but I really think that they did a good job here. I was afraid that they would go crazy with the visual effects for the spaceship battles or the practices games, but they did not. Instead they kept in turned down and focused on the story instead. Another topic that had me apprehensive was the relationship between Ender and Petra. They could have focused it on a romantic affair that really was not there in the book, but again they kept to the book and kept it a purely platonic pre-teen friendship. Well done!
Yet another point that had me sceptical was having such a young kid as the leading part of the movie, but the choice of Asa Butterfield turned out to be just right. He did a really good job playing Ender.
You might also fear that a major star like Harrison Ford would take over the movie, but despite delivering a flawless performance his glare was turned down to avoid outshining the rest and it worked.
Translating a book into a movie will always be of some cost to the story and of course they had to cut out some parts. Peter Wiggin, who had a very large part in the book, is barely mentioned in the movie and a lot of the emotional challenges for Ender were also turned down to a minimum. Personally I think they should have left out the ending (trying to avoid spoilers here), and I know it is a part of the book as well, but I just do not like it. It is a bit too spacy for me and does not fit with the rest of the book/movie. That is just my opinion.
In general I think they did a good job however, and if you like sci-fi, this is a must see. If you have read the book either as a kid or an adult, you have probably already seen the movie at least once.