I've just dipped a toe in that water with a Contax 25mm on a Canon/Contax adapter. First impressions on the Contax versus the Canon 24-105 4.0IS and the Canon 24 L T&S is that so far the Contax is bringing up the rear. I appreciate the stellar Zeiss performer is the Contax 21mm, but before spending £1000+ on I wanted some re-assurance that there was a general Zeiss benefit.
Regarding putting Hassie lenses on a Canon, I can't see any advantages, and I say that as a Hasselblad enthusiast. There are no realistic wide-angle Hasselblad options, Canon already excells with their longer lenses, and most Hassie lenses are heavy enough to strain a Canon mount!