Let's gather some impressions.
After my first hour with the demo there's three things I noticed:
1. Demo Mode: it's great to have this demo mode! I wouldn't be able to decide such a buy just based on some online sound demos. What went a bit on my nerves is the rhythm of the demo audio dropouts: you can hardly play a phrase and get a context impression before the next silencing phase comes in: a little longer pauses between the dropouts would really be helpful without changing anything about the protection.
2. Positive sound impression:
This thing really cuts through! I tried different kinds of (in part loud) contexts, and whatever I did: this piano will never be buried in a mix. It's a vivid, powerful rock and jazz piano especially.
3. Negative sound impression:
While the basic sound is fantastic, dynamic and powerful, the way in which attack and sustain go together (or rather don't) is weird! All the variations have a mighty (almost too mighty) hammer attack phase, just to fizzle out to a thin sustain sound immediatley after the attack phase. This sustain is definitely TOO thin from my view, especially with the higher notes, and more drastically so with the bright version of the sound.
This thin sustain just doesn't sound natural, and I wonder how that was obtained. I once had the chance to try an Imperial Grand and don't remember such a low sustain sound from that at all! No idea how that happened to this otherwise great sounding sample set.

My question to Korg staff would be: is there any chance to change the sustain for an update of this sound, or is it how it was recorded and will stay like that forever? To me, this question might decide if I want to buy it or not.
It's really a pity: such a great basic sound! And then this ultra-thin sustain...