I've got a Dell Axim V50x
It's a full pocket PC.
I'm a network engineer by trade. I guess my phylosiphy is, if I'm going to drop a couple hundred bones on an electronic device I'd like it to do more than only play MP3's like the most iPod's. (which is overpriced and under featured IMHO.)
A pocket PC is going to cost more over all, but you get the ability to add on soooooo much more. It's almost limitless.
You can get flash cards up to a couple GB which will hold about 250 songs per GB depending on your bit rates, etc. And the cards are swappable.
My example that I'm seriously considering implementing over the winter is....in the boat and my "toy car,"...I already have head units that have an aux in (or you can get an FM modulator too, but a direct connection is better.)
One movable cradle for the Axim and I have an MP3 player / GPS (with addon hardware,) / full contact list / e-mail / video player / navigation software that talks the direction to you (through the car stereo) / fishing hot spot software that's GPS enabled, etc, etc, etc.
It's just a matter of what you want. The iPod does it's job well...it plays mp3's. but its realy an over priced player due to good marketing.
One other thing to keep in mind is...solid state flash memory type players are more stable and will use less batt. power than a spinning Hard Drive type device. They are usualy a litttle smaller in size and capacity though.