I don't think it'll detract much at all. The war with the buggers is essentially a MacGuffin. (Spoilers ahead for anyone who's managed to miss this for the past few decades.) The real heart of the story is in the stuff that happens to Ender at Battle School, his evolution from innocent child to military genius to embittered pacifist and his relationships with the other characters. The twist ending is great because of the way it reflects everything that's happened to Ender so far, not because the surprise is just so surprising.