Upload & Sell: Off
I've been using the ZF 35/1.4 since soon after it came out. My only other near comparisons are the CV40 SL-II, CV 35/1.2 on crop, and ZM 35/2.8 on crop bodies. My favorite of the bunch is the 35/1.4 on an FX camera, first the D700 and now the D600.
At a pixel level, it's not going to be as "sharp" as the Sigma until around f/4-5.6. It also has a fairly wicked amount of green/purple fringing at high-contrast interfaces wide open. It's one heavy bulky beast to carry around for the day. Finally, stopped down, one must really stop down to f/11 to get the corners to catch up with the center due to field curvature, though you lose acuity centrally in the frame once you go above f/5.6. In that regard the CV 35/1.2 seems to be better for a fast lens, an excellent lens at f/5.6, see Ron Scheffler's comparison to other rangefinder glass.
For me, the benefits of the lens are the dedicated manual focus helicoid, the accurate (on my camera) hard infinity stop, and the "rendering." My Sigma 50/1.4 had buttery bokeh, and a buttery look in the focal plane as well, if it managed to acquire focus. I prefer the out of focus rendering for both up close and mid distance subjects from f/1.4-2.8 to most if not all samples I've seen from the Sigma, let alone the other lenses I've used.
If I knew I was going to have both a DSLR setup and mirrorless (NEX in my case) setup running in parallel, and the Sigma was already out at the same time as the Zeiss, it would have been a tough decision for me. But at this point the Sigma doesn't offer enough to make me sell the Zeiss.