I've got a Contax Aria 35mm film camera and several Zeiss lenses. I bought an adapter for my XT and... it's so-so. The good news is that it does the infinity focus correctly (I had an earlier adapter that would not - anything past 50 yards was blurry no matter what). The bad news is that after you focus the lens a few times, the motion begins to loosen the adapter. I love the idea overall, I just don't like the adapter. I guess that's what you get on ebay though, I wish I'd had a hands-on with it before purchasing. I have an idea to modify and fix the adapter but have not tried it yet.
I for one don't mind the manual focus at all, it takes almost no time to get used to. As to the quality of the lenses, I know they are great but in my very limited time shooting with them on the Canon I just can't tell a lot of difference between them and my better Canon equipment.
Canon's 50mm 1.4 was not any worse than the Zeiss 50mm 1.4, nor better. Equivalent enough to not bother.
Canon's 17-85mm (which I got rid of) was *much* worse at 50mm than the Zeiss 50mm 1.4 though, maybe not a huge surprise. I was never happy with the lens though even before I tried comparing it.
I have a 17-55mm IS Canon lens now, but haven't done a comparison, I've been totally happy with the 17-55, and since I don't care for the adapter ,I haven't tried it in a comparison against the Zeiss lens. I predict they'd be indestinguishable when tested at similar settings.