+1...the 300mm f2.8 IS II & 400mm IS II are more likely be used for sport, however, I agree with Scott. Both lenses are quite amazing to shoot portrait - even a simple flower shot below
Another thing that is very very nice about this lens when shooting wide is the bokeh. Very special.
+1 ...... Love the bokeh on this lens. It seems strange, but I often use this lens for a portrait lens for close up work.