first of all it uses a standard laptop hdd which is HOT SWAPABLE.
Cool, you can change out the hard drive while it's on?

Seriously though, I still have one around that I use to record my sets, and it works fairly well for that, but generally, for a portable music player, it's rather bulky, and the user interface is
really clunky compared to the iPod's; for, y'know, actually listening to music (and podcasts and audio books) when I ended up getting an iPod a little over a year ago it was a definite step up. I mean, seriously, who in their right mind would think it's a good idea to make it ask you if you want it to remember where it's at in a song or whatever when you're turning it off instead of doing so automatically? Braindead stuff like that.
Aside from the much better UI, though, the chief advantage for me was that I was able to take the system of smart playlists I had setup in iTunes on the go and have it keep last time played, rating, etc. updated on iTunes whenever synching it, not to mention the ability to update the smart playlists without hooking it back up to the computer.
Soooooo much nicer.
