the aperture clicks of the Zeiss ZM lens feels more solid than those Leica ones. Several years ago, when the Zeiss Planar 50 was out, I handled it, the impession was poor, it felt very light, the letters were printed on the body etc.
this year, well, it should be last year now, I handled them again, (have not examined the 50 planar yet), they feel wonder, the letters are inscripted on the body, body is of excellent weight, smooth focus movement, the hood mount is impressive, shadow of contarex lens, fond memory. standarized filter thread of E43 or E46 for most of them.
as to performance, it is different from modern leica asph lens, they are top of "analogue" lens, not excessively sharp or contrasty.
I currently have c sonnar 50 and biogon 25, planning to add biogon 35 this year.
the Leica might very well be the top either on test chart and A/B comparison, but I doubt very much whether you will see the difference in real life, and in any event, the Leica better be good for their selling price.
the only advantage for Leica optics is the large aperture lens, the 35 asph summilux and 50 asph summilux, they have no competition in sight at any price range (have to mention the cv nokton 35/1.2 and c sonnar 50 mention as being the very good ones), others have seen their territory encroached upon by voigtlander and zeiss lens.
