Dedicated portrait lens?

Unless you really like macro I would not use the 100L for portraits in your case. I also have the 24-105 and the 100 macro non L, for portraits besides the sharpness of the macro there is little difference between the 2 in the separation and bokeh department. You will see a much more noticable difference when you compare your f4 lens to the f2 of the 135L or to the f1.4 of the sigma. I was researching a possible upgrade to the macro L for the IS and the weather sealing, but once I saw the bokeh of the 135L I fell in love and I got it instead of upgrading my macro. If you want I can do a quick comparison of the 3 lenses wide open.

It would be interesting and appreciated if you could

I would suggest something like the Sigma 85mm f1.4 if you are shooting everything from headshots to full body. If you doing headshots only then the 100L would be a really great option!

I tried the 85f1.8, 100f2 and 100L. I found the 85f1.8 too short for my taste. I really liked the 100f2. I decided to buy the 100L to replace it as portrait/macro lens combo. Although I enjoy the 100L and I find it much sharper than the 100f2, I prefer to shoot portrait with the 100f2. Bokeh and everything else look better. I am not sure how to explain it, maybe just the f2 making a big difference.

Since I didn't have a model available I used my trusty tripod. The space between the tripod plate and the base is about a head, so just imagine a pretty girl smiling at you. Since this all about background blur I chose an angle where there are objects at different distances, I tried to keep the framing as close as posible but there are slight variations. The lenses used are the 24-105L f4, 28-105mm f3.5-4.5, 100mm macro (non L) and the 135L f2. Just for reference I included the 135@f4 as well. I hope this useful.

28-105mm f3.5-4.5 and the 100mm f2.8 macro

And finally the 135L f2 @f2 and @f4

basically it comes down to focal length. i use both an 85 and a 135.

i suggest starting with an 85. with an 85 you can always stand closer, at 135 you are kind of stuck standing back.

here are two images. one at 85 f1.8 one at 135 f2. the 135 has a richness and creaminess that is unmistakable. the 85 however, at $300 used, is no slouch.

OP never mentions what he considers to be ...

1. what is a 'portrait' shot"...

   full standing portrait,
   3/4 length, waist up,
   head & shoulders,
   tight headshot

2. at what distance ordinarily will his subject be at...

   studio portraiture (8-10' clasically), or
   what I call the 'shy photographer shooting candids (unposed)'

How about a 70-200 2.8 IS ? It is a very versatile lens and can be used for lots of other stuff not only portraiture...

Wilt and Moncho, thanks to you both for some very well thought through responses that took some real time.

Good question on what I want to consider a portrait. I think full body to mid-chest and up. Since I don't have a dedicated studio, most will be some "on location" type of shot, so I leaning towards the Sigma 85 1.4 to give a bit more cramped quarters flexibility.

Thanks everyone.