Upload & Sell: On
My impression of Zeiss lenses is that they are very good, but not better than Canon. Canon's offer a lot more options such as AF and IS, plus the wonderful TSE lenses that Zeiss doesn't offer.
My experience was with ZE 50/1.4 (negative experience), ZE 21/2.8, ZE 35/2, Contax (CY) 50/1.4 (outstanding), 80-200/4, 135/2.8, Contax N 24-85 and 70-200 (on Contax N1 film camera). Certainly not the full range, but representative of the qualities heralded by Zeiss devotees. I found the wide angle lenses to suffer from severe light fall-off (particularly the 21/2.8) resulting in over-exposed centers, or very dim sides. Also, these lenses were not sharp edge-to-edge, as is often claimed, until smaller aperture over ride the curvature of field and vignetting. They are generally well made, but excessively heavy, especially the 21/2.8 and 35/2, when compared to the 24 TSE II (similar weight and bulk, but far more flexible) and the 35/2 IS (smaller, lighter and far more features).
I was also disappointed when the Zeiss magic I saw on images in the Alt forum Zeiss thread failed to materialize for me, until I found out the images were almost all (relatively) highly modified in post-processing. Once I applied similar techniques, my Zeiss images looked "better", meaning more dramatic and sharp with tools in LR. Incidentally, so did my Canon images! I found I could get similar or better images (to my liking) with the Canon lenses. So the Zeiss became an unneeded luxury, tying up photo funds I could spend on other items. I sold all my Zeiss lenses, except for the Contax 50/1.4 and don't miss them.
If I were buying a Nikon D810, I'd seriously consider repurchasing the 21/2.8 and 35/2 in the ZF.2 mount, I feel the lenses would be of more value than similar Nikon offerings. (But as I've said many times, if Canon lenses could be mounted on Nikon with full compatibility, then, no, I wouldn't bother with the Zeiss lenses again.)
I can't say don't buy the Zeiss lenses -- I did! If you are curious and have a bunch of cash on hand, go ahead and enjoy. They are certainly high quality lenses, no question about that!
Good luck on your Zeiss exploration!