The following were processed from raw using Photoshop ACR set at its most neutral settings - i.e. everything at zero with no sharpening and no noise reduction.
First to get some idea of centre sharpness.
Secondly to get some idea of edge sharpness.
Thirdly to get some idea of CA and fringing.
In terms of other wide angles that I have used these results are pretty good. I gave up on the idea that I would get the same performance as a standard prime lens with a wide-angle zoom, or even a wide angle prime some time ago. *
By creating a Photoshop ACR preset for CA and fringing and adding a little sharpening I was able to get an excellent result.
I find this very acceptable. Certainly the best overall result I've got from a super wide-angle lens, which I define as wider than a 20mm full-frame or 35mm equivalent.
*The only super wide lens that I've ever seen as equivalent to a standard prime is the Nikon 14-24mm f/2.8 zoom. I've never used one but all the reviews and descriptions seem to indicate that its super-sharp across the frame with virtually no CA or fringing. However it is huge and weighs more than most cameras, so its not an option I've ever considered.
In terms of raw sharpness in a slightly narrower lens the Zeiss 21mm f/2.8 I use on my Leica M9 takes some beating, but suffers from vignetting and a magenta cast.
While neither of these things are particularly hard to fix, it just adds an extra problem to the processing.
In the not too distant future I will probably be using my Leicas and m4/3 only. Since my last wedding the Canons are unused. If the Panasonic GH2 is what I expect it to be then I will become "DSLR free" So my wide angle options will have to be for the Leica M9 and my Olympus and Panasonic m4/3 cameras. I'm not planning anything else for the Leica. There are the same issues of vignetting and the magenta cast with many m-mount wide angles, and though the in-camera presets lessen the problem I gather from what I read that they don't always eliminate it. I'm happy with the 21mm in terms of sharpness and can put up with the corrections I have to make.
For m4/3 the Olympus 9-18mm looks to be the best option at the moment. Today the sun is out so hopefully I can get a days shooting in with it to make a more informed judgement.