I just borrowed this lens for a few shots. I have the EF-S60mm already, so it was interesting to see the difeerences. I can't really see any, except for the obvious: working distance is increased; (good or bad) this lens fits all Canon bodies and it feels a bit heftier. Quality wise, they seem to be on par. I'd really like to get my hands on a Sigma 105mm Macro and see how that fares.
I have the Sigma 105 mm macro and a friend of mine have Canon 100 mm macro. In my opinion they are comparable in image quality they produce. But then again I am wrong most of the time ;o)