I use Canon 14mm v2 and really enjoy the coverage, but for the best results I stop down to f8 or f11. Images shot @ f2.8 - f4 have disappointing results in the corners.
Wait, you mean images at 2.8 when focused on a distant object will be soft in the corners.... that's because your focal plane is far away.... for sure.
Focus ON something in the foreground, in a corner (you must MF for this as there are no focus points in what we consider the corners), and even at 2.8 the resolving quality of this lens is insane on a 1ds3. Incredibly sharp.
I ordered Marks adapter for the nikon, as I was sold by the review... but tired of waiting and got a 14mmII. This lens is just plain nuts! By no means discrediting his review... but the images Ive taken the last couple days with the 14mmII lay to waste the results he shows on his site. The CA is much better controlled in the files Ive gotten, and sharpness runs to the corners easily. I suspect that Marks copy is, sadly, well out of spec. Either than or I got incredibly lucky.
When I do get the adapter, I'll do some side by sides as well, for the sake of science and such (lol), but I assure anyone the 14mmII, while as expensive as all get out, IS indeed a high resolving, sharp rendering UWA prime.