One of Canon's great lenses. Even though I own an 85/1.2L and have tried some of Canon's great L lenses I have to say that my best pictures were taken with this lens. It is incredibly sharp wide open but once you stop down to F/8 it becomes nearly unbeatable. It is also very versatile - it's a macro which goes to a true 1:1 magnification, it's a great portrait lens, it's great for a short telephoto for street photography.
I have also compared this lens to the Tamron 90/2.8 Macro and the Sigma 105/2.8 Macro. In terms of image quality, both the Tamron and the Sigma are nearly indistinguishable from this lens.
The Tamron however is a variable aperture. The aperture gets smaller the closer you AF. Both the Sigma and Tamron have front elements that extend with AF. And both are significantly slower to AF than the Canon (and people complain that the Canon's AF is slow!). The build quality of the Tamron is on another planet. It feels like a cheap consumer lens. The Sigma is a bit better but both are not as solid as the Canon.
If you compare it to the 85/1.2L for portraits then the 100/2.8 is more clinical at F/2.8. All detail is mercilessly resolved. The 85/1.2L has a certain flattering look for portraits that cannot be described. But this is a VERY MINOR nitpick. This lens certainly served me very well for portraits until I got my 85/1.2L.
Build quality is on par with L lenses. If I blindfolded you and gave you the lens to handle, you would think it is an L. If I showed you the images, you would think it is an L.
In short, this lens is superb! It is affordable, sharp, and versatile. It has a permanent place in my bag for this reason.