I have two pairs of MDR-7520 that were purchased a couple of weeks apart from each other. I liked the 7520 so much that I bought a backup pair when it looked like they were being discontinued without a replacement being introduced. (turns out they only discontinued them in Europe)
When they were new I tested them both to check that they were working correctly, and they sounded almost identical.
The first pair has been used daily for about 18 months now, for hours at a time. The other has been sitting unused in a box.
I've pulled them out of the box a couple of times to do comparisons, and there is a clear difference between them now.
Bass is tighter (more detailed, with more impact) and treble is smoother.
Before this, I would probably have agreed with the opinion that burn-in doesn't exist - I certainly don't think it does for solid-state components or things like cables.
But with dynamic drivers, it does seem to make a difference. People have also said that wear on the pads can change the frequency response, so that may also affect the results.
Sony MDR-7520 is now on the front page of Head-Fi as a gift guide feature. It has been discontinued in the EU but has it been discontinued in the US?
I think they were removed from sale in Europe due to laws on how loud headphones can go. With the low distortion, high power handling, and high sensitivity of the 7520 you can turn them up to very high volumes which could potentially be damaging to your hearing.