I was a skeptic once, but awhile back my friend bought one of those Audio-Technica IEMs and was complaining to me about the lack of bass. And I was kinda surprised because two other classmates had the same model and I thought they were just fine. Then the next day he let me listen to his, and I was shocked. It was the thinnest sounding thing I ever heard, below even the worst laptop speakers I've ever heard. Now remember I had nothing invested in this. I wasn't expecting anything since it wasn't my earphones. I blamed the lack of seal first, since he was using the smallest size silicon buds, lo and behold a few days later he stopped complaining and said that there was SOME bass now. Here it got interesting because my friend had no knowledge whatsoever of earphones except that iBuds suck and IEMs = good. Naturally I started thinking that wow, if he could tell a difference with no knowledge or opinion on the issue of 'burn in', there has to be something really obvious.
So the next day he brought it to school, same buds and all, and holy hell I experienced my first incidence of 'night and day difference. The previous time I had already tried fitting the things in my ear using all sorts of methods, even pushing it so far in that the buds were in danger of getting stuck there, but even then they sounded horrible. When I tried them again, I didn't even bother to attempt at a perfect seal, just stuck them in straight, and woo. I have to admit, I haven't detected burn-in prior to this. Not with my iBuds, EX51, HJE50, ATH-A700, and countless other ~$50 headphones, zip nada. Not even with the expectation of burn-in effect and the placebo effect that tends to come with it did I detect any changes in those headphones and earphones,