Well, in the absence of any other replies here's my view. I shoot a lot of macros with Canon gear, especially the Canon 100mm f2.8 USM Macro and the MP-E 65mm Macro lenses. I also have Tamron 60mm f2.0 Macro and Sigma 180mm f3.5 Macro lenses.
The Canon Macros have the edge in image quality, but not by a huge amount and I am happy with the images I get from the Tamron and Sigma Macro lenses.
Equally important to the lens you're using is the lighting. I shoot a lot of hand-held macros "in the field" and a good flash setup is essential.
Check out my galleries (lots of macros) at :