There is a 'review' and general chit-chat about over on engadget
iRiver's AK100.
The MQS encoding process gives the music a refined and pure sound.
Iriver has partnered with HDtracks to provide users with albums encoded with Master Quality Sound.
So MQS is a codec, not just a marketing buzz word

Why oh why could then not just stick with FLAC and WAV; and HDtracks should know better too. That said at least this device does support these format as well.
Regards,
Carl
PS Oddly, the specification page only lists: "Audio codec support AAC, FLAC, MP3, OGG, WAV, WMA" no MQS.