John - With all respect for the experiences of the other members who've posted here, I borrowed a 2x to use on my EF 70-200mm f/2.8L IS for a daytime air show, and I was reallydisappointed in the results. Maybe it was just a mismatch between my particular body (40D), my copy of the lens and that particular copy of the 2x. For the first time ever, I could see CA in my pictures. The contrast and color were lousy (granted, there was a lot of glare). A guy next to me was using a 5D with the 100-400, and he e-mailed me some shots. I don't have his images anymore, but the side-by-side comparison of our almost-identically framed shots of a YF-22 Raptor dramatically illustrated the difference in our equipment. I wasn't shooting wide open, either.


One other note: the 2x/70-200 combo yielded its worst results at full zoom (2x200=400). If you're planning to use it there, I would go with the 100-400.


Also just remembered- I read or heard somewhere that the 1.4 and 2x weren't really intended for use with zooms.