The 1.2 I had came on a Leica M3 (with a Leitz screw-to-M adapter). I used it with T-Max 3200 for existing light, stuff like a band playing in a dimly lit bar for example.
I traded the lens even for a Contax II with a collapsible Sonnar (f:2, uncoated) and got pictures I liked much better under the same conditions with the same film.
Well, that's all well and good but maybe someone else would prefer the other look?
I've shot stuff with my Jupiter-8 Sonnar clone that I wish I had lugged the Canon around for.