As I recall, their existing 50mm f/1.4 does not gather very good reviews on a FF body, so its time to make something thats better.
Photozone gives it 2-1/2 stars for optical quality, while the much less expensive Canon 50mm f/1.4 gets 3 stars, and it has autofocus as well.
"Some years ago we analyzed the Zeiss Planar T* 50mm f/1.4 on an APS-C DSLR and the results were truly impressive there. However, on a full format DSLR the performance leaves a bit to be desired actually. The lens is capable of delivering great results at medium aperture settings - no doubts - but it disappoints at f/1.4 and f/2 with a rather poor border/corner performance and generally reduced contrast. Depending on the use-case this may or may not be overly relevant (e.g. portraits) of course. The light falloff is on the high side here but that's a rather normal characteristic for such lenses. Lateral CAs are well controlled and not overly field relevant. The quality of the bokeh is pretty good although the weak contrast may spoil the game at f/1.4.
Just like the rest of its family the Zeiss lens is built to the highest standards. Some users may complain about the lack of AF but this isn't really a significant flaw for a wide-angle lens. The focus confirmation is available in the viewfinder (ZE version) and in very critical (close focus) scenes Live-View can give you the needed guidance. That said, it remains a bit of an anachronism these days. The price level is quite steep so you really have to have a itch for such classic lenses."
I've seen some mediocre reviews of the 50mm f/1.4, and I rented it a few times just out of curiosity before I bought any ZE glass and it was much, much better than I expected. It's damn sharp and I was very pleased with the image, just not wide open, but heck my 50L isn't that great wide open. I still ended up opting for the 50 f/2 which is sharp as a tack at just about every aperture, no complaints on that one.