Canon 28mm f3.5 or f2.8 on Leica M - Anyone one using one?

My experience with both the Canon's has been very positive, especially the 28/3.5. The 28/2.8 was really pushing boundaries optically when it first came out, but it is a beautiful compact gem of a lens that does a decent job, especially stopped down a little. The Avenon/Kobalux 28/3.5 is a modern and superior lens optically to the similar Canon, but I have found the build quality to be fairly poor.
I had a Canon 28/3.5 on a Leica CL that I liked a lot. I used the whole VF to frame and it worked well for me.

I currently use a Kobalux 28/3.5 on my M 240 and I must say I find it superior to the Canon though the old Canon lenses were second to none in build quality.

Hope this is of some help.
I'm going to try a 40-43 step up ring to see if that eliminates the vignetting wide open...... since i have some 43mm filters.