I've got a Ripflash Plus. Looked long and hard at the Sony MDs, and have to say it was appealling that the PoGo was only about a hundred bucks rather than $250 or so for the MiniDiscs.
Also, the Sony MD Walkmans available last spring didn't have any USB or path to digitally upload to a PC so one had to use the line out to the soundcard line in--which meant two A/D conversions, one in and one back out to the device after massaging the sound in GW.
Here's a link that might be interesting if you're looking at the MD:
http://www.minidisc.org/uploader_table.html
That all said, for field recording the MD [or some as-yet untried device?] may have better fidelity in the higher frequencies than the PoGo RipFlash Plus. Apparently, on recordings made with the RipFlash using the analog line-in jack, the PoGo MP3 recording codec clips frequencies above 17KHz even though it samples at 256Kbps/48 bits. Happily, the Ripflash will play back to 24 KHz --without frequency clipping-- MP3s that have been recorded (on my PC) at the GW/LAME codec at 320Kbps and 48bit sampling, then downloaded to the RipFlash via the USB cable. [whew!]
(In other words, if you want [the most exquisite] mouse and lipsqueaks and the high range stuff, you'll get better fidelity recording to computer [and then downloading to the Ripflash], rather than recording directly to it, at least the RipFlash Plus model I've got).
I know birders use the Sony MDs a lot. But I don't know of any who have looked on a frequency analyzer to see if the Sony codec clips high freqs in it's compression. [Ditto the iPod and iRiver.]
The iRiver harddrive gizmos look interesting. Also Apple just yesterday announced some new DIGITAL wireless link that connects their iPod to a home stereo that's coming out next month. (though iPods may not record yet? Dunno.)
LionHo
[hope that's a little clearer now?]