Not just on GAF, but the world in general. The iPod consistently outsells the competition; in terms of marketshare, it has the biggest piece of the portable digital audio player pie.
You're obviously not going to listen to it all at once, it's a matter of convenience. There's something "liberating" about having every CD you own stuffed into a little device roughly the size of a deck of cards, and have room for even more. There are no CDs or minidiscs to swap, I never have to plug it into my computer unless I want to add even more music, and it doubles as a giant USB hard drive so I can take data between work and home.
It's being able to cram those racks and racks of CDs into your pocket, and listen to whatever you want, whenever you want, and have it available with about two button pushes and a quick scroll.