This thread is over 2 years old and Biggles summed it up pretty well. The MZ-X500 had the POTENTIAL to be a great board (and easily Casio's best effort since the MZ-2000. But... the keybed is awful, probably the same one they used on the CT-X3000. And the MIDI implementation is half-baked, so you can't even use it as an arranger MODULE! What a shame.
Still holding out for the CT-S900 (think about a CT-S500 with a larger screen, style editing, full 5-pin MIDI implementation with "chord judge," etc.,. and you get the idea!)