A month or so ago I got to try out a canon 100 2.8 Macro lens. Its a nice piece of glass to be sure, and I put it up against my truly great Tamron 70-200 2.8 as some sort of comparison. 

You can see the HD Vimeo Version HERE

The canon lens is a very good performer as could be expected. It focuses from infinity to nearly inches in front of the lens which is great. Even more amazing is that this is an internal focus lens ! Most macro lenses extend themselves like crazy in order to accomplish this task. The big downside is that the focus rotation of the lens is incredibly short, less then 90 deg as I remember. This means that when focuses close, wide open, where DoF is under an inch, almost breathing on the lens will mean the difference between being in focus or not. In the test video, I left some of my focus searching in there so you can see how tight it is. Realistically, canon should have at least 180 deg of focus rotation in a practical lens. I'm not saying you can't get the hang of this glass, its just very very touchy !  

Canon 100 2.8 Macro Lens Test VS Tamron 70-200 2.8 using 550D from Steve Oakley on Vimeo.

I now brought out my very close focusing Tamron 70-100 2.8 not expecting too much. To my surprise, at 200mm I got shots quite close to what I was getting with the canon 100 2.8 macro. The Tamron lens doesn't focus nearly as close, but when working its closest distance at 200mm, its about the same as the canon at 100mm at 2 feet or so. Its very impressive to say the least. I could not quite get as close with the Tamron, but I'm not complaining  either. With an extension tube I could easily match the canon. Check out the test video and see for yourself.