I have a 100-400 IS L, 24-70 L, 70-200 f2.8 IS L, 16-35 L, and I just ordered a 24-105 IS L to replace my 28-135 IS (non-L).
I still use the 100-400 if I need the "reach" on my 1DsMkII and 20D. The 70-200 L is sharper and faster, but with a 1.4 Extender, they are about equal.
I have many fine images shot with the 100-400, that's why I hate to part with it even after reading many posts about how this lens is not up to par with the 1DsMkII's resolution capability.
North Wales, PA