It does look to be a pretty good lens as evidenced by the posted image.
However, at a focal length of 12mm one would be looking for softness in the corners, and, as best I can make out, most of the high-detail content of the image in question is toward the centre of the image.
Obviously, you have the luxury of perusing a full-size raw image at 1:1 in Lightroom or whatever raw-converter you use.
This particular lens would be on my wish-list were it to be an ace performer - my experience with such wide angle zooms to date has not been fantastic - but I live in hope....
I am away for a month or so from next week, perhaps, when I return I will look up my favourite retailer in Brisbane and test out this lens...
Tony Jay
There's plenty of details in the corner, particularly the fine detail in the lower left. Obviously it's not as sharp as a 35mm or 85mm prime, but I wouldn't expect it to be. The lenses I would compare it with are the Sigma 12-24, Canon 11-24, Nikon 14-24 and Voigtlander 12mm. It also has a similar angle of view to the TS-E 17mm shifted all the way to the edge. Here, it seems similar to the Canon (if not a bit sharper) and comfortably beats the rest. It even seems to beat the shifted TS-E 17 at the edges (and certainly has a lot less CA), even after downsizing the shift-stitched image to an equal resolution, although obviously the shift-stitched image has greater centre resolution.
It certainly seems to hold up to the standard seen in testing at Lensrentals, FredMiranda and Kasson's blog. I haven't used it at f/4, and would certainly expect the corners to be softer there, but I can't see myself using it at that aperture anyway (at least not for any subject where corner sharpness matters - if the corners are out-of-focus anyway, it doesn't matter how soft they are) and it's likely no worse than anything else at that focal length.
This was just a quick test near the front of the store to make sure it wasn't a total lemon - I wouldn't do a proper test handheld at ISO 400 anyway.