The 100L is a quite impressive portrait lens. DOF is nice, Bokeh is lovely, it is compact and light. The AF is also fast and reliable (on 1D mkIII and 60D, with the AF limiter switch used).
However, I rather use the 70-200 II as it offers just a little worse IQ for a lot better focal range. IS is about equal. I do event photography most of the time, where portraits are included, but they are not the first priority. But if I would go on a creative session, when I have time to look for the perfect angle and distance, I would use the macro for portraits. The white guy is so heavy, that shaking hands are becoming a real issue after a few hours.