The 1.8/50 is a very usable lens. I had one coming with my first Canon-P (later black series), this was very sharp but missing contrast due to fungus (looks like fogging) which could not be completely removed. It seems that this kind of "fogging" appears very often at this particular lens-type. Now I have a earlier chrome version, clear as a star and much better mounting. The chrome outline keeps better over the time. As far as I know the optical design is the same. The first slides look very auspicious. Attention to flare with side-light. All these 1950's 1960's lenses performs better with a hood. If you are common with Voigtländer modern coated glass you will notice that straylight is no problem for them. For Canon's it is!
The 1.8/50 chrome has virtually the same compact size and weight as the Canon 1.5/50 (Sonnar type), my favorite allround lens (same filter -40mm- too). It's seldom I really go for the high speed hammer Canon-7 &0.95/50 lens... it usually stays in the showcase and looks imposingly.. 😉
I'm not familiar with the 1.2/50 but I would suggest it's not an allround lens either: weight/size, shading the finder, more difficult to focus. This was the first Canon Planar design with 7 elements/ 5 groups from 1956. The 1.8/50 (6/4) will tend to much less flare. From stories I heard, I wouldn't expect a better performance at f/1.2 than with the later (1961) 0.95/50 wide open, which was build with newer glass-sorts. I think for the price (the 1.2/50 goes for less than a 1.4 or 1.5) it's an excellent lens.
