I remember a thread on avsforum.com where one individual has claimed he could hear the difference between cheap and expensive cables. Someone on the forum offered to travel to his house, do a double blind test, and post the results. The individual agreed, and the test was performed. The results of 20 tests were something like 12/8, where 12 was the number of times he mistook the $5 cable for the $900 one.
Well, that was kind of expected, however the most interesting part was that despite this clear evidence, the individual still refused to change his mind about importance of the "high quality" cables. It was a bizarre and eye opening experience for me to follow that development.
Some other people, however, see it as 8/20 you can hear the inferior product, they think that they are not peasants to get 8/20 feeling of inferior product, so shut up and take my money, cause $900 cable will guarantee 0/20 of hearing bad cable, totally worth it. nuff said
I would probably argue for a more expansive definition. If you have to listen really, really hard to hear the enhancement piece of equipment gives then it's a waste of time and money.
But hell, I grew up listening to music recorded onto a C90 through a $40 walkman.