I have to say, I'm more than happy with the 12-24 I've had for a while now.
Pity I can't say the same thing about the D800E tho!

Only problem with the Sigma(which everyone will experience is very far edge performance at the wide end.
It's only at about f/16 where it starts to come good.
But it doesn't really bother me in that I don't pixel peep like that for images shot in that manner.
That is, when shooting wide vistas, I tend to prefer that the image as a whole makes the image look interesting, rather than pixel details that really mean nothing!

it's so8unds like it's going to be a whole lot more expensive than the current model(US$1500 .. close to AU$2K!) rather than the current AU$800-900-ish.

My main issue is that I've changed the focus matte screen on the D800E for a coarser type, which is great for fast lenses .. bad(to very bad) for slow lenses.
When zoomed just a tad past 12mm, (say) 14-17mm, it's aperture slows down to f/5-f/5.3, and with my focusing matte, it darkens massively.
(you have to see the 500/8 to believe it!).
So the f/4 aperture is my main point of interest, and considering Sigma's current examples of their Art and Sport lens image quality abilities .. I'm assuming there will be some increased IQ ability too.

In saying that tho, the current 12-24 is amazingly sharp in the centre across all focal lengths.
This isn't to say that this is an advantage in any way .. just a comment on that it can produce some great results.