Yes, it would be definitely interesting to see how it performs. Nikon's 24-120/4.0G VR
was not my fav for a high resolution FX-body like the Nikon D800 but it performed reasonably and there was no real alternative except for the surprise performer the Nikon 24-85/3.5-4.5G VR
- but that lens does not have the same reach at the tele-end.
Now the Sigma 24-105/4.0 OS is about 10% shorter at the long end. But if it can make up for the loss of reach with a clean image (and a well-working OS) I'd certainly prefer it to the Nikon.
Looking at street-prices, the (not yet available) Sigma (for Nikon mount) has reached around 800 EUR (incl. 19% VAT) and the Nikon 24-120/4.0G VR is still hovering around 950 EUR. So an almost 20% price difference might also help sell the Sigma. For Canon mount the Sigma prices already dropped well below 800 EUR to around 750 EUR. But that is still more expensive than the price of Canon's own Canon EF 24-105mm 4.0 L IS USM that has been steadily dropping below 700 EUR in the last quarter.
Testing at slrgear suggests that the Sigma looks almost perfect on a Canon D7 but corner performance on a 1Ds at f4 is definitely not good but f5.6 seems to make the FF-corners usable. But I hope to get a chance to get a first hand impression and test it on a Nikon D800 and compare it to the other medium range alternatives.
P.S.: again a case where Sigma delivers the Canon mount before the Nikon mount version of a new lens.