I think it's a vast improvement on the 800 in these areas:
1) key feel - it's fantastic
2) size/weight ratio - perfect size in my opinion.
3) sounds are improved ... more DNC ... cleaner guitars
4) TC Helicon says that the internal units are almost identical, but only designed by TC - Korg assembles them on site 100%.
5) better touch screen and more ergonomic layout - so many cool buttons
6) MP3 recording - did the 800 do this? I forget, and did the 800 have the chord sequencer?

For the size/weight/keyfeel and sounds alone - I'd rather have this on my stand. I've owned the 800, and it was not of the same caliber.

Here are 2 demos.. one made before I adjusted the mic a bit and one with a better master EQ. (I think)