The problem with many books-turned-movies is that they lose a big chunk of what made the books great. Relationships, characters, sequences un-necessary to the plot, etc.
The book Ender's Game is mostly built up of Battle School battles, rivalries, and friendships. However this movie was built mostly upon Harrison Ford.
While that's not necessarily a bad thing... any movie can be built upon Harrison Ford. But that's not why we love Ender's Game.
Despite all this, I still love it. I love it for what it is; it is a fun movie.