I think we will have to wait for a little more "real world" feedback from independent reviewers/users.
I am quite sure the IQ will be very good - Sigma have already proved their ability to make top quality glass with their other Sports/Art products. My concern with this particular lens is the AF performance. Certainly Sigma have made significant strides forward in recent years but they still have to make their lenses match someone else's camera.For many (most?) users they have already achieved this but with the sort of tasks asked of a 500 F4 then even a small reduction in performance could blow the deal.
I would love to give one a thorough work out as I will probably be looking for a 500 F4 in the not too distant future as my current gear is getting heavier as I am getting older!
Fingers crossed!