Okay, actually I don't. I like the interface and having all my music in one place and how it works with the iPod. But there's the problem. ITunes only works well with the iPod, and vice versa. I wouldn't own a different portable player, but if I did I would want it to work with iTunes instead. And vice versa.
As a matter of fact I do have a different portable player – my cellphone. Which I'm using to type this blog entry actually. It happens to play mp3s, after a fashion (biggest problems being it uses Real player which has a slick but awkward UI, and it requires a different set of headphones with a tiny stereo plug.) But of course it doesn't synch with iTunes, unless someone makes a plugin.
Then again, if iTunes and the iPod interface protocol were open source, maybe Apple wouldn't have done so extraordinarily well selling the little bastards.