Here's a tantalising opener if I've read one:
“Carl Zeiss ZM lenses is an amazing phenomenon is (sic) photography: despite their excellent quality and ergonomics, most Leica M shooters do not even know about them. The main reason for their relatively low popularity, besides snobbish attitude of “those with the best glass in the world”, is that at any focal length Leica optics surpassed Zeiss ZM lenses in speed. That is until now…”
This review, written by DPReview forum member, Irakly Shanidze, is salted with well thought-out images (and the above freakish thing); and though it doesn't prove anything intrinsically different, or better, about the Zeiss, it is a worthwhile read.