Since it appeared, through successive generations of redesigns (e.g. iPod Nano), the iPod still remains overall the dominant device in its market.
But it isn't just the device alone that makes iPods so successful. Apple offers the device AND also provides the major source of content. Like many others, I have some issues with the functionality of iTunes, but it provides a constant revenue stream for Apple and that is at the core (no pun intended) of their success.
Before I bought my first iPod many many years ago, I used to buy MP3's from Wal-Mart's web site, but I don' know if they even offer that any more. Their only other real competitor in terms of music is Amazon. Once in a while there is something really obscure that is only available in a digital format from Amazon, but for the most part, iTunes rules the music market.
As a side note (and totally off topic), that is what is so surprising that these roles are exactly reversed when it comes to books. If Apple and Amazon would ever collaborate (which won't ever happen), they would completely be in control of content.
[/rant] OFF