Just a heads up to those thinking of getting this lens, you might want to wait a little.
Most of the reviews I have read have been done on cameras of lower resolution than the D800/E and the only exception has only so far looked at close range usage. I have been using a brand new copy extensively for a few days now, intending to review it, and I have found that it is less impressive at the edges than early reviews and MTF charts had led me to hope. This finding is clouded by the fact that mine appears to have a mild decentering, making the right hand side slightly softer, but neither edge qualifies as great on this copy. I have sent samples to Sigma to see if they regard it as within spec but I suspect that, for those awaiting a really good landscape lens in this focal length, this might not be it. I suspect that field curvature is to blame but not until I have a certified good copy can I be sure.
It is remarkably sharp on centre, even wide open. But to give you an example, I was so surprised at the edge performance at far-mid and landscape distances that I made a comparison shot at F4 on a Sony RX1, which I regard as the gold standard for edge and corner sharpness in full frame mid wides, and then I down-resed the Siggy/D800 file to the same pixel dimensions and it was, literally, no contest: sure there was still a touch more detail on centre from the Nikon rig but the Sony easily took the edge trophy.
I have repeated my tests of the Sigma at closer range and it seems somewhat better there (excluding the right hand edge weakness) but still not what I would call critically sharp like the Sony is.
One snowfall does not a winter make, and I will report back on what Sigma have to say, but despite the fact that this lens is very nice on centre, has fairly low distortion and aberrations, good flare resistance and is very sharp indeed wide open on centre, it is not currently looking like the answer to 'sharp across the field' needs. For me, that's still the Sony...