Well, there are 21/2.8, 25/2.8, 28/2.8, and 35/2 in the Biogon series, besides also Planar 50/2. I am considering to buy one 50mm for my R2 that's why I hope to see any comments on the Zeiss lens.
I've once seen pics from a Japanese using the Plannar 50mm/2. The color renderation and the sharpness is superb.