A very good point. No one knows if ships will ever reach speeds like that described in SF novels. But honestly, I've read hard SF novels that have more questionable science. Stephen Baxter, an acclaimed hard SF author, has stories where ships bounce around at FTL speeds and a hard bridge connects the moon and earth. In the same story the two main characters are the same person a few years apart, existing in the same time. The novel is Exultant by the way, and it is a great read.
Ender's Game is a great SF novel. Exultant is great as well.
Since you brought up fantasy, I'll make another point as well. There really isn't a lot of difference between the two genres. Fantasy is just more honest about not being possible. The technology used as plot devices in most SF novels is so far from our current tech level that it might as well be magic for all the chances of it ever becoming as commonplace as it is in those novels. SF authors (and I love SF, don't take this the wrong way) wrap their magic in nuts and bolts to make it seem possible, but ships travelling that fast, or FTL, is honestly nearly pure fantasy, as you pointed out Sarakoth, so I concede that point. But it doesn't make me enjoy the novel any less. They're all fantasy novels as far as I'm concerned, and they read better that way as well.
I know this contradicts my earlier post trying to defend the science behind the novel, but I don't care. I post stupid things some times (well... a lot really and chances are I'll think this post is stupid in a few days).