I have the M-Rokkor (front element cleaned by FocalPoint); the Canon 28/2.8 (CLA by DAG); the Canon 28/3.5 black/chrome (haze cleaned by Essex); and the Skopar 28/3.5.
The M-Rokkor is my current favorite. Sharp, less contrasty than the Skopar, but still "modern" in flavor, fast enough for a 28mm and quite small for an f/2.8.
The Canon f/2.8 comes out in bright sunlight, as it's much less contrasty than the others. It's also less sharp than the others, including the Canon f/3.5, but that's probably variable with individual sample. The Canon f/3.5 is also very nice, and a bit easier to find than the f/2.8.
The Skopar is all around a great lens. Maybe even sharper than the M-Rokkor to my eye, but that could be just perceived sharpness due to higher contrast. Very small and well made. If you shoot film (and can control contrast in dev), and if you don't need the extra speed, try to get the Skopar. It's maybe harder to find right now than the M-Rokkor (strangely enough), but it's a keeper and trouble-free.
I used a CV Ultron f/1.9 briefly. Great lens, but twice the size of the largest of the others we're discussing here.
Ari