The 100/2.8 is a macro lens, meaning that it is capable of reproducing an object the size of the image sensor, known as 1:1 magnification. Despite the macro designation, the 100/2.8 can focus to infinity like regular lenses. As such, it is appropriate for use as a portrait lens.
Aside from the macro feature, the 100/2.8 Macro stands out among the 80 or so Canon EF lenses for its resolution and contrast. This should come as no surprise, because "macro lenses are among the best lenses there are for 35 mm format.