I checked out Sony's cool looking players myself. But what everyone says here on the forum, I found to be true also. The supporting software is very buggy, according to reports. And they don't really play MP3's, they "transform" it to Sony's proprietary format. . . ok, if the software works for you, but bad news if it doesn't. The IPod's proprietary format is easy to work around, especially using Apple's own superior iTunes software (I'm using the Windows version), so if you're looking for a HDD unit, I wouldn't let that stop me.
I own an iRiver flash player (512Mb). It's not expandable, but other than that, it's a fabulous device. If they're HDD unit is as good, I'd recommend it too. And if they are supporting Ogg-Vorbis. . . BONUS for Linux guys, and very cool of iRiver!