No color artifacts besides chromatic aberration, which is mostly fixable (trickier in a shifted lens than in a centered one). The corners just get soft. In cases where it's just sky or the equivalent in the corners it won't matter, so how far you can go will be contextual.
It's a lens you have to learn your way around. Aperture is important, and the sharpest aperture will depend on the degree of shift and the degree of important detail in the corners. In a worst case situation, with over 6mm shift and detail across the image circle, I find f16 to give the best results. In other cases it looks great at f5.6 to f8.
As an unshifted lens, the sharpness and overall look are spectacular. Better than any other wide angle lens I've used in small format (I have not used the Nikkor 24 1.4g). There's nothing special about out-of-focus rendering. I don't care, but bokkeh people probably won't be impressed.
It's a slow lens in practice. Manual everything, including aperture stop down. Easy to forget until you get used to it. Very limited automatic modes in the camera. I just use it in manual exposure and focus in live view.
The build quality is as nice as anything I've used in any format. Better than any Nikkor I've used.
I believe that Leica commissioned this lens from Schneider, who simply recycled the 28mm f2.8 digitar they already had lying around. They just created the shift mechanism, and added a floating element for close focussing (a curious choice ... would make more sense to me in something like their 85mm tilt/shift lens that's designed for studio use). I don't think this design was ever considered a world class performer in MF, but was possibly the only retrofocus design Schneider had ... necessary for sticking it on a dslr.