Our auditory memory is lucky to be accurate over a matter of minutes. Over hours or days - it's a pure guess. I'd be interested if you could compare a broken in X5 with a brand new one (volume matched, same tracks and settings). Personally I think you'd find very little difference.
I agree with Cannikin - my personal experience suggests that it's our wonderful brains adjusting.
That is quite possible. Hard to say for sure without directly comparing new X5 (A) and old X5 (B). However, when I say that I hear a difference I am IMMEDIATELY comparing the difference in sound of my iPhone 4s and X5 A/B when I first got it versus now and I notice a change. (smoother highs and MORE bass). However, I didn't accurately match volumes, even if the files are the same.
As someone trained in perceptual sciences I realize that this was not properly tested, and it is possible that it is all in my head.
I guess my point is that even if it is in my head, it was enough of a change to turn me from a mocker of SS burn-in to a believer in SS burn-in.
Either way I love the sound of my X5 enough to make it worth overlooking the UI shortcomings compared to my iPhone 4s. And that is all that really matters.