I find the 5D isn't a body to use for high magnification shots like the ones I take with the MP-E65 but it's absolutely perfect when mated with my 180L for closeups of larger subjects....bees, butterflies and flower studies.
It provides a fantastic bokeh, isloation and tones...far better than the 30D's.
To sum up....sharp technical macro work with a crop sensor but creative closeup shots with the 5D and 180L
Bob