I have never used an extender and wonder if using one would make the Canon + extender a real pain in the butt. Since money is not my problem what do some of the forum members think on this subject?
Background: I sold my 1.x4 vII extender in June with my 70-200/2.8L IS vI. I sold the zoom because I was purchasing the vII model under a rebate offer, and the buyer wanted the extender, too, which I didn't use much.
I didn't have any issues with the extender's performance, I just didn't like the hassle of removing it and remounting it as the need arose. I usually only use focal lengths greater than 100mm when I'm hiking; therefore, out "in the wild," where any lens changes are opportunities to drop, damage or at least sully equipment.
I bought the 100-400 II in December, so I now have more reach with it than I did with the 70-200 + 1.4x. I'm not ruling out the possibility of someday picking up a 1.4x III, but won't do so unless I absolutely need it.
IMHO, your degree of satisfaction with using extenders (with the exception of the built-in one in the 200-400) depends largely upon how frequently you need / want to mount and unmount them.