I'm not a professionist and I like to take some portraits outside, so I've preferred ef usm 100 f 2.0 for its capability of isolating the subject due to its focal length and it's aperture. beside it has no distortion at all and its sharpness in the middle and at the borders is not worse than 85mm.
Actually I could simply say that I've preferred a longer length for a metter of taste.
I recently bought the EF100f2. I considered the EF85f1.8 but I figured the EF100 with that extra little bit of reach would make a handy concert lens as well. I thought about the EF100mmf2.8 macro but decided against it being that bit slower.
So far I've mainly been using it between f2.5 and f4 and the results are very impressive. Certainly a worthwhile step up from my 70-200f4 as a portrait lens. I've got some shots I took at f2 to have a look at soon to see how it is wide open.
Bought it from 7dayshop.com for 270 quid (it's an American import).