I think I mentioned this before... in 1978 I bought a Canon Serenar 28/3.5 "Ultra Wide Angle Lens". It must have been their very first design, from the Korean War era. It was unsharp at any aperture or focus distance.
Some years later I had a black & chrome Canon 28/2.8. That one was much better. I traded it away because I decided that I didn't care for separate viewfinders.
I have the 28 f2.8 and the later black-and-chrome 28 f3.5. (there is no f2.8 black-and-chrome)
Wide open, they are both sharp in the center but not at the edges. They get better as you stop down.
The f2.8 is just a little better than the f3.5 at all apertures.
If you can find either for a good price, go for it.
*There is now a Voigtlander LTM 28mm lens. A bit more expensive but its a brand new lens.