I'll pitch in for the D-CJ01.
Merits: Higher quality sound than the SlimX, lower HP output but very good line out - superb for amping. Takes AA batteries (Alk, Nicad, NiMh) and charges both Nicad and Nimh as standard while being not much bigger than the SlimX. Display on main unit or optional better-looking remote (RM-MC11EL). Hardly ever skips.
Demerits: Less bells & whistles - no EQ and too much 'mega bass', expensive with a remote, won't play everything like the Slimx (copes perfectly well with CBR & VBR files on the whole, but the disc has to be closed).
Bottom line: Best MP3 quality from a PCDP - CJ01. Max bang for the buck - SlimX.
Firmware upgradability is not a huge issue - all they've done to date is to fix bugs on the SlimX! MP3Pro may be on the cards for SlimX (extends quality of very low-bitrate files), but anything else on the horizon will require too much horsepower.