sdsr said:
...portraits at a wide aperture you might find yourself hooked by what you can achieve that way (and not just on people - flowers, pets, buildings etc. can all benefit) and wish the 24-105 were faster than f/4 (I seldom use my 24-105 for that reason, excellent though it otherwise is).
Indeed. I use the 24-105mm for portraits when I have a studio-type backdrop, in which case I'm using f/8-10 with strobes in softboxes, and it's great:

EOS 5D Mark II, EF 24-105mm f/4
L IS USM @ 105mm, 1/200 s, f/9, ISO 100
You can get a nicely blurred background with f/4, but you've got to be
really close to the subject, and ideally have a relatively distant background:

EOS 5D Mark II, EF 24-105mm f/4
L IS USM @ 105mm, 1/160 s, f/4, ISO 100
But for portraits (since that's the title of the thread), I'd really recommend the 135L over either the 24-105L or 100L Macro, assuming you're shooting FF. It's great for melting the background in action shots, too:

EOS 5D Mark II, EF 135mm f/2
L USM, 1/1600 s, f/2.2, ISO 100
Of course, if you want to
really melt out a background, try the 85L - it's great for portraits on both FF and APS-C:

EOS 5D Mark II, EF 85mm f/1.2
L II USM @ 1/60 s, f/1.8, ISO 400

EOS 7D, EF 85mm f/1.2
L II USM, 1/2000 s, f/1.6, ISO 100