Here's what our old buddy Rockwell has to say:
I've only used these lenses in Nikon mount. I wasn't impressed. The optics were identical to the real camera-brand lens, but the mechanics were second rate. Is Zeiss kidding? Other third-party lenses graduated from ribbed-metal focus rings by the 1970s.
These Zeiss-branded lenses aren't made by Zeiss; they have been made by Cosina in Japan, the same people who have been making third-party lenses for people like Vivitar for many decades.
One bit of caution is to understand that these lenses are not made by either Canon or by Zeiss. They are third-party lenses completely unaffiliated from Canon. Canon does not sell them, does not approve of them, and certainly has not provided electronic, mechanical or any other design information to Zeiss.
Third-party lenses have the annoying habit of occasionally having incompatibilities with some features on some cameras, so be careful to test any lens you get on every camera with which you intend to use it. I've had some Tokina lenses that some of my Nikons won't even recognize as being mounted!
These Zeiss-branded lenses, at least the ones I've used in Nikon mount, are competent optically, but not better than Canon lenses, and the Canon lenses autofocus, weigh less, and have much better manual focus feel than the ZF Nikon-mount versions of these lenses. I doubt the Canon-mount variants will be any different, and I'll let you know when I get to try these in person.
http://kenrockwell.com/zeiss/ze.htm
