I have a Pioneer 740MP ($230 from thezeb.com) and it's a great unit for listening to one mp3 after another, but it's a bit slow in the switching. As far as I can tell, it doesn't remember the index/layout of the mp3 cd and has to re-read the index everytime it switches to a new file - which makes it slow if you're skipping around. Also, this unit doesn't fast-forward or rewind very well with mp3s and the optional changers cannot be loaded with mp3 cds (not sure if any retail models can, but I did test it out with the 1270? changer).
Overall, I'm still happy with my HU as I really never intended to listen to mp3s on it (audio fidelity snobbery I learned when people were ripping like crap) but the radio1 essential mixes (2 hr long sets) have forced me to reconsider.