I read the book, so I can't un-read it before watching the movie. However, I did my best to consider what it would be like if I didn't. And I felt all of the characters were incredibly flat. At no point was I convinced that Ender was particularly intelligent (at least no more than the other kids), nor did I see that he had any particular insight into anything. He just won at stuff because his name is in the title of the movie.
I believe this movie would have been much better if it followed the book less closely. They didn't do a great job with the initial setup, and the monitor, etc., and would have been better to cut it entirely. Same with Ender returning to Earth. Cutting details like this would have given more time for some actual character development during battle school.