I can't help but to utter these words which are true heresies for the Leicaphiles ...
The physical look and feel of the 21mm Biogon ZM matches or exceeds that of my Elmarit 24mm Asph. The aperture & focus rings are buttery smooth or should I say Leica smooth. And, the aperture ring has no play whatsoever (very impressive).
The clicks of the aperture ring are smoother than any of my Leica lenses --- the sound of the clicks are more damped/subtle and, to my ears, sleeker.
The coatings on the front element looked authentically real, just like my other Hasselblad T* lenses.
In short, the mechanical and physical feel of this ZM lens is pretty Teutonic and up to par.
In the next week or so, I'll run it thru a few rolls and see its pace optically.