I may share some images later, but here are my thoughts:
Presently I have both Sigma and Zeiss. Both are fantastic! Sigma beats the Zeiss in sharpness and microcontrast @ f/1.4 (the Zeiss stumbles just a bit from a veiling haze that robs some detail). The sharpness advantage and the Zeiss veiling haze disappears at narrower apertures. My feeling is the Zeiss beats Sigma in the gorgeous bokeh department - and the Sigma has a very very pleasing bokeh!
We can insert a joke about the Sigma focusing faster than the Zeiss, but I had an interesting experience recently: I was doing a tripod-based landscape shot and wanted to manual-focus the Sigma. With AF switched off I used Live View to pin my focus and for some reason the Sigma seemed jumpy when manual-focusing - I get the focus almost just where I wanted it, and then the focus plane would seem to jump over/past the precise point I was aiming for. I realize most everyone is going to AF the Sigma nearly all the time, but the thing is, the Sigma makes an excellent landscape and architecture lens and I prefer to manual-focus with those scenarios. I never thought I'd complain about a lens not manual-focusing well (I have no such problems manual-focusing my Canon lenses), but my Sigma doesn't seem so great at that. Don't misread me here, this is a very minor nit, but I found it an interesting learning-about-my-new-lens experience nonetheless. 
After getting the Sigma last month I was so impressed with it that I had planned to sell my Zeiss, but I can't bring myself to follow through with those plans because the Zeiss is so dog-gone special. So for now I'm just a happy moron with redundant 35mm f/1.4 lenses that are both incredibly good.