I am in the honeymoon phase of shooting the ZM1.5/50 Sonnar lens and am admittedly head over heels in love with it. The rendering is very rich, sharp enough - but never too sharp, and the contrast and color rendition mate so well with the M8 that I have yet to even shoot on black and white film with it.
On the negative side, it does show off a lot of chromatic aberration at larger apertures. This appears to be a side-effect of the spherical aberration that gives its distinctive "glow" at the same apertures. Thus it may not be a contender for the ultimate all-rounder 50, but at the moment I am quite tempted to learn how to work around that issue. I love all that it does well enough to possibly learn to adjust.
Long intro complete!
So now I'm curious to know if Zeiss' entire range of "C" lenses share these characteristics, or if they're limited to the Sonnar lens. Both the 2.8/35 and 4.5/21 are Biogon designs - do they render similarly to the Sonnar?