My 4th generation iPod works great under WinXP. There shouldn't be many differences to using one on the Mac, escept that you will probably need to install iTunes, which comes installed on all new Macs.
Realplayer also works with iPods, but it will convert all files to MP3 or AAC. Apple isn't too happy about this though. There are other programs that work with iPods too, like Ephpod and Xplay.
Just a note as this is a common misconception, AAC is not an Apple propriatary format but the planned replacement for MP3. The Fairplay DRM used for iTunes Music Store files is an Apple propratary format though. The Fiarplay is just wrapped around the AAC file.
As for the difference between the iPod and other hard drive players, for me it was the simplicity of the iPod. Others may be cheaper or have more features, but very few, if any, have as clean an interface. This includes both iTunes to organise your music and the interface on the iPod. The difference in price and battery life aren't that big anymore either.
Sony's new music players look good, however, based on my experience with their Minidisc players, I will wait a while until I see the limitations they place on them. They also have a habit of using their own music software that is far more complicated to use than necessary when compared to either iTunes or Windows Media Player 10.
My iPod does what it was designed to do very well, that is play music. It also performs very well as an external hard drive.
goates