I can't comment on the newer Zeiss lenses but I'm using older Contax Zeiss lenses on my Canon body with an adapter and the quality of the shorter focal length lenses is very impressive. I did some very basic tests comparing my Canon 24-105 and Contax 35-70mm f3.4 and the Contax was miles better than the Canon.
Some of the newer Zeiss 35mm lenses are reportedly better than the older Contax lenses while some are worse.
I've tried 2 different Zeiss/Contax 28mm f/2.8 lenses with an adapter on the 5d2, and neither could focus to infinity. I assume the adapter was too thick. So, some lens-adpater combinations are clearly better than others.
I've read lots of reviews of these lenses, but some of what I read suggests to me that my problems are not unusual. Samples of comments regarding adapters and these lenses include that thinner adapters are needed for wide angle lenses (why should it matter, because it's more critical with them?), and Zeiss lenses seem to have more depth of field (of course they would if they're not really focused on infinity). There seems to be some disagreement as to whether the 28mm Contax lens will clear the mirror. Well, on mine it cleared -- just would not focus to infinity. Note that I don't see comparative shots at infinity in these reviews.
So, be sure to deal with people that you can return the goods to.
I've put together a selection of MTF curves that I used for my searches at http://www.paulroark.com/BW-Info/MTF-charts.jpg
This includes the "missing" Photodo Zeiss-Contax 28mm f/2.8 MTF.
That lens did perform well in lab resolution tests, but it was very hard to see all that much difference between it and the Canon 35mm f/2, center or edge. (I tend to crop out corners of 35mm frames.) In the field the Canon 35mm f/2 easily won even at f8 and f11, probably due to the focus problem. The Zeiss lens sure did have relatively sharp foregrounds, however.
The Zeiss 28 had some odd shooting characteristics. The contrast appeared to be greater, and highlights tended to get blown out. While it showed darker corners at 2.8, at f 11 the edges seemed over-exposed.
I'm an old Zeiss fan and prefer good manual focus lenses, so I look forward to the ZE line. But I'd be wary of these older optics and the adapters.