The 100-400 IS is the worst lens in my kit

Skip Middleton writes ...

Worst, in this case, is a relative term.

Yes, I can pick between the 70-200 f/2.8 L, 300 f/4 L, 400 f/5.6 L,
100-400 L IS and 500 f/4 L IS and the 100-400 is definitely the weak
sister (relative?) in this litter.

The 100-400L is a pretty good lens

Now that you have a full frame digital body shoot a grey card or even
toned sky with it at 400 f/5.6, f/8 and f/11 and check for vignetting
.... I have one of the "good" ones per Roger Clark's tests (at least for
center resolution) and with a 1.6x body like the 10D vignetting is OK
wide open, with a 1.3x body (1D Mark II) I get too much light fall off
at the corners wide open and need f/8, and with a full frame body (1Ds)
I have to go to f/11 to get rid of the vignetting. This is the only
non-wide angle lens I own that has a vignetting problem like this so to
me it doesn't qualify as a "pretty good lens". None of the other four
tele lenses do this. I can even shoot the 500 f/4 L with a 2x wide
open with no problem, but not the 100-400 even without a converter.

... but nothing else will do what it does ...

That's the only reason I haven't dumped mine ... when I fly to Alaska
with space and weight limitations this lens seems to worm its way into
the bag beside the 500 f/4 L even though ideally I'd rather take the
70-200 and the 400 f/5.6 (last trip for instance I took the 28-70 f/2.8
L, 100-400 IS and 500 f/4 L IS and the bag still weighed 36 lbs with
two pro bodies ...) ... but the IS means I can shoot from a platform or
a plane or handheld or whatever with a couple extra stops and the
100-400 zoom range is very handy.

Bill