I read the novel when I was in college on a flight home. Loved the book. 
The movie was quite good, it didn’t really go into all the strategies the book describes, but you get the idea. There were some visual problems with some of the sets I found distracting. There is also a scene Harrison Ford has a giant crumb of something on his chin, highly distracting. Other than that, solid performances all around.
If you read the book and enjoyed it, you should see it in theaters. If you haven’t read the book, its still a solid rental for sci-fi lovers.
Michael,
I read the book about 20 years ago and have been waiting for a movie version since then. I have a signed copy of the whole series of "Ender" books and Orson Scott Card talked at the signing about 15 years ago that a movie was in the works, but he would not let it be made without being true to the book.
I think he kept to the story well enough that some scenes I just smiled because I knew what was about to happen. Predictible but awesome at the same time.
The only thing I can complain about is that if you never read the books, you did not know about Ender and Valentine showing their intelligence by controlling "Popular Opinion" by the articles they wrote on the public nets. The same goes for some of the scenes in the battle room. Ender’s genius was not really explored in the movie as well as it was in the book.
Otherwise, great film! Asa is one great actor for his age!