A couple of alternatives would be the Cosina Voigtlander 90mm (an f/3.5, I think) and the Leitz Elmar-C 90/4, available on the used market. The CV lens has a good reputation--I've not used one. The Elmar-C, originally made for the Leica CL, is very sharp and very well-made--I do have one of these. Both are relatively inexpensive. Leica said the "C" lenses may not focus properly on M-series Leicas but mine focuses perfectly on two M6's and I've never heard of anyone having any problems with "C" lenses on "M" bodies.