My copy of the Canon 50mm macro is quite exceptional on my 1DS3 with no fringing even wide open, and very sharp. I recently compared this lens to the 50f1.2L and that was an eye opener, lots of fringing and not particularly sharp until f4, and even then not great. I think that in general, especially with the likes of Canon and Nikon, sample variations are more likely and with Zeiss and Leica less so.
This week also tried on a friends D3 was a Zeiss ZF 50f1.4, which at f2.8 and close up was scary sharp, about the same as my 55f2.8AIS, but the Zeiss contrast was considerably higher which gave some added punch but no more resolution. The Zeiss ZF 50macro is meant to be quite something and can be used with a Canon adapter, but i have no personal experience, only a discussion with someone i trust who is selling his 50f1.4ZF to buy the macro.
Regards, Gary.