I'm curious as to why more use isn't made of these as well.

I've had a WDTV for a good while now and it seems like a no brainer solution for a music server for cheap if all you need is an hdmi or optical out. I found the WDTV sound quality to be very good, neutral with the newest firmware. I thought that w/ digital out that there should be no difference in sound but apparently WD added digital gain to the units early on. After complaints they released an update and you could hear the difference in digital out.
That said I just purchased a Patriot Box Office because WDTV downsamples hi-rez to 48khz and I want to output straight to my new dac w/o any alteration. So far the PBO sounds kind of brittle w/ raw hdmi out. I don't know if media players need break in or what (seems doubtful), but I'll give it a few days. I remember the Asus O play I demo'd a few months ago having the same sound.
I wish there was more info and transparency about these players. It's hard to get info and what's out there seems strictly geared for the PC/video/torrent geek crowd ( I mean seriously just how many video/networking formats does a person need?)
The Patriot seems a good deal because of the rebate, internal hd space, and you can buy a wireless dongle for cheap.