Soon after I purchased my 70-200mm IS II, I twisted my wife's arm into posing in the back yard. The attached photo was shot with a 40D at 135mm (actual focal length), 1/400, f/5 & ISO 400. You can count each eyebrow hair with no difficulty. I couldn't be happier with the sharpness, but Terri doesn't like the fact you can see she isn't 30 years old! (She was 63 at the time the photo was taken.) The 40D has no fancy AFMA, and only 9 focus points, but from my perspective it is tack sharp. I couldn't be happier with the lens on any of the bodies I have.
Had the 2xTC III for use with the 70-200 II, but felt I just ended up with a cumbersome heavy combo. Probably should have chosen the 1.4 EX instead also giving me f4 and not 5.6. Had to send the lens to Canon as the lens new out of the box had a defect rubber gasket at the lens mount (part missing). Also I am not that impressed on the sharpness. It is great but not great as in fantastic.
I currently have the 2xTC III + 70-200 II combo and it works well except in low light (as expected). I had previously the 2xTC II and the IQ was inferior to this combo. Still, you pay for having the zoom flexibility instead of prime 400mm 6.5L higher carrying weight