135mm work well on a full frame indoor portraits, and is a great length for crop & FF outdoor portraits. A fast 135mm is a great length for indoor sport photography on a crop body such as basketball and volleyball.
I own a FF body, and I use the 70-200L f/2.8 IS II almost the exclusive since I purchased it early this summer. While my old 70-200L f/4 IS was use at around 50% of the time. And, I have to say 70% of the shots from those lenses are at 160-200mm wide open. (200mm is a bit long for portraits but it gives great compression)
I have use my 100L f/2.8 IS for portraits in the past, but I rarely use it now since I own the 70-200L f/2.8.
I also have the 24-105L that I rarely use because I found that 105mm is too short of a focal length portraits and f/4 on this lens is too soft for my liking.
IMHO, each to their own, but I would love to try shooting with a 300-400mm lens at f/2.8-4 for portraits.
Full Frame