Ive been using the 12 Heliar on my M8 for about a month. I find it to be very sharp, with some falloff at the corners, but nothing so significant that it can't be fairly well overcome within Lightroom.
Attached are 2 images. These were taken in the cockpit of an old Blue Angels Jet. The first is not adjusted at all. The second is the same file with only 2 LightRoom adjustments under the "Lens Vignetting" area:
1) Amount +50
2) Midpoint 30 [which is actually moving the slider -20]
While these are not the most sophisticated tools for doing this, it shows how the file can be made to look pretty good without a lot of time intensive work.
BTW- this image was done by just setting the M8 on the cockpit seat, setting the Aperture to F8, putting the Shutter Speed on "A" and pushing the shutter button. The exposure was 32 Seconds!!!
I'll upload 2 more images in another Reply for your review.
All in all, I really like this lens. I hope this helps. George