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135L is my favorite portrait lens on my 5DII; I find the telephoto helps me get a more spontaneous look, since the subject is farther away; with my camera-weary teenagers, this is a huge advantage.
And, even though my skills may not illustrate it well, it is sharp, sharp, sharp, wide-open...the photos below were casual snaps around the dinner table at Christmas...the relatives were a bit more relaxed when I wasn't pointing a giant white 70-200 f/2.8 at them...
Just last summer, I sold my venerable 100mm macro in order to get the newer 100L with IS; I've found myself more and more shooting hand-held closeups, and the stabilization is a tremendous help. If you shoot more tripod-mounted macro/close up than hand-held, the 135L with extenders could be a one-lens solution for you...for now .
(edit: I just realized all of them were shot with a "1.3x crop sensor"; oops. Well, it works well with a Mark IV, too; I get upper body frames from even farther away...I also second Ian's suggestion; it is hard to go wrong with any brand of the 100mm macros...)
Canon EOS-1D Mark IV 135 mm f/2.0 1/160 sec 6400 ISO -0.7 EV
Canon EOS-1D Mark IV 135 mm f/2.0 1/160 sec 2000 ISO -0.7 EV
Canon EOS-1D Mark IV 135 mm f/2.0 1/160 sec 1250 ISO -0.7 EV
Canon EOS-1D Mark IV 135 mm f/2.0 1/125 sec 1600 ISO -0.7 EV