i had this 'MP3' Walkman. great little device: you pulled out the knob (heh) and twisted it to change tracks, volume, etc.
what was absolutely shite was it couldn't actually play MP3s: you needed some terrible Sony software to encode your music to a proprietary Sony format.

got some Sansa microSD one after this, could play FLACs and everything. not as cool but it was a godsend, just mass storage file transfer like a USB stick (imagine that!)