I would use it at the long end for portraits in a heartbeat. The softness is very flattering for anyone over 20, and from 150 - 200mm it conflates the distance between nose and forehead, giving VERY flattering perspectives. I believe it was Scott Kelby who said that if you photograph a woman with anything less than a 200mm you will lose a friend, even though you have a beautifully sharp photo, because of exaggerated perspective around the nose and cheeks and forehead with any lens less than 200mm in focal length. The same is true for men as well as women, unless you are after brutally sharp, grungy photos that reveal every blemish, facial hair, skin discolouration and flaw. Those photos are "in" these days, even of old people, but I prefer to keep friends.
That being said, I hardly ever do portraits, but in my limited experience, 200mm yields the nicest looking portraits of all.