See my 73-page Guide and Review of the Leica M9.
Frank G writes:
I’m interested in a 21mm lens myself to compliment my 35mm and 75mm lenses. I thought about the Zeiss ZM 21mm f/2.8 for a while but now Leica introduced this lens. So, now I'm thinking... will this Leica lens at $2995 be that much better than the Zeiss 21mm 2.8 ZM at $1425 w/ lens hood? hummmm...I can't wait to read your review.
Note that both lenses require an optical viewfinder for framing the composition. I prefer the Zeiss ZI 21mm viewfinder, even with the Leica M lenses— less expensive and better setback for the eye, at least without glasses.
As I document, the main issue ZM lenses on the Leica M9 is focusing accuracy— Zeiss still insists on adjusting focus for the ZM lenses for film cameras (a good way to make their lenses look soft). See that page in my review for details, including confirmation from Zeiss.
The other issue is the correction of color shading and vignetting towards the edges and corners. While vignetting is no big deal, color shading is a very big deal unless shooting is for black and white purposes. Leica addresses the lens correction in the M9 via 6-bit coding of each M lens, and while ZM lenses can be coded to trick the M9 into applying its corrections, there is no exact match for the ZM lenses, with results varying by lens from very good to ugly.
Given both these factors, I cannot at present recommend ZM lenses for the M9. Accordingly, my lovely pin-sharp 35/2 Biogon and 25/2.8 Biogon are both for sale.
As far as sharpness for the Leica 21mm f/3.4 Super-Elmar-M versus the Zeiss ZM 21/2.8 Biogon, both perform at very high levels The bottom line is that these two lenses are so close in performance that the tiniest focus error is the real issue, not the lens itself.