@jackhifiguy - I can't say that I've experienced any changes in a cable from burn-in, but I can also say that I've never paid any attention to it on the few cables that I've upgraded.
I have experienced clear differences with one power cord upgrade (1 of 3) and also an interconnect upgrade. The interconnect improvement was significant enough that I opted to upgrade a second time almost immediately.
One of my friends is a bit of an audiophile and also an electrical engineer, so you can imagine having to try and convince him that I've heard a difference. I know it perplexes him a bit because he doesn't want to believe it, but has also listened to my system since I first purchased it and knows that I don't claim an improvement/difference with every change, so when I equate the different to a similar order of magnitude as tube rolling he can't just discount my opinion.
Have you ever experienced a change in sound quality after changing any type of cord in your system?