Huh. Well, at 15w4d our u/s tech was quite confident she saw little boy parts. At 16 weeks, our amnio results revealed that he is, in fact, a boy. So for us, u/s was right on.
ETA: And actually, I've never really known anyone for whom the u/s was wrong. I mean, I've heard stories (haven't we all?) but it was nobody I know.