timmmay: My girlfriend didn't seem to care about the difference in sound quality between a pair of $30 headphones and the HD650s. I don't know if she couldn't hear a difference or just didn't care. Since she doesn't seem to have any hearing problems I suspect the latter.
I'd say the latter too, I think my wife would definitely fall into that category. When she figured I was serious about my upgrade, she did make a genuine effort to hear, but day to day, so long as she can hear the music, that's good enough for her.
I on the other hand, have a much greater interest in music, I love the feeling from being blown away by the quality of the reproduction etc.