I never use mine for portraits, my 135mm l is so much better.
I agree that a macro is not a dedicated portrait lens (thus I got the non-L version) - but out of interest: what's so much better about the 135L? sharpness wide open? bokeh?
All of the above plus F/2 aperture.
Lack of IS may bother some, but for portraits, IS is not a big issue to me.