I've been using
the JP-8000 for about 10 months now, and although, as with all Virtual Analogue synths, the poly is limited, it does kick out some truly Phat sounds.. I use mine mainly for lush analogue strings, but it is capable of some quite crisp leads, and if you're after some weird sounds, then many oddities can be coaxed out of it with little effort..
I've only recently started editing custom patches on it, and with a decent patch editor a libray of unique sounds can be made quite easily. Having said that, the main thing I do love about the JP8K is the number of knobs and buttons you can fiddle with on it. Lots of experimentation and many a sounds has been created from just fiddling away.
I've not heard or played the Korg, so my opinion is somewhat biased. I would however state that it's not a master keyboard and is very good as an addition to a studio rather than being the centre of one. Maybe someone on here who has the Korg can comment?