The piano sounds are kinda horrible. I'm still trying to figure out how to connect my roland XP to my Mo6 via midi, so that I can have a decent piano sound.
The organ sounds are pretty good. They are missing a few sounds from the ES8. They also dont sound as phat as they do on the tritons, but I like the way you can use the sliders as drawbars on the "fully" patch. And I'm lovin that "swishie" organ patch, its just so soulful and perfect for that west coast sound. I got mine for $650 on craigslist, well worth the price.
If you plan on layering and what not, then get the ES6, the mo6 has too little polyphony to do anything too demanding.