DanB -

I used to own a Yamaha MU90. I bought it used from SamAsh for $150 a couple of years ago. I midi'd it up to my Technics digital piano and honestly was never very pleased with it. The sounds all reminded me of my old DX7; nothing seemed particularly authentic. It ended up sitting on my Technics untouched for probably a year before I traded it for a Line6 Guitar POD.

It was very compact, 1U high and a 1/2 rack module, but I never fell in love with it and couldn't recommend it.