The 40mm M-Rokkor is slightly more desirable than its optical twin, the 40mm Summicron-C, since it is multicoated and takes standard 40.5mm filters. The Summicron-C is a little weird in that respect.
Originally Posted by Lee L
There seems to be such a kneejerk tendency to try to divert people away from Leica optics that I sometimes wonder whether there might be a tiny bit of non-objectivity involved. I happen to have a 50mm Summicron (latest version), and it is for all intents and purposes optically flawless. There certainly may be better lenses out there, but it would likely be a test bench superiority, not something which would be noticed in practical use.