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 have the 2XTC III as well and I think I should've chosen the 1.4TC III instead.
Body: 5DIII. Prime Lenses: 15mm f2.8, 100mm f2.8L IS, 35mm f2 IS, Extender EF 2X III.
Zoom Lenses: 16-35mm f4L IS, 24-70mm f2.8L, 70-200mm f2.8L IS II. Others: Flash 580EX II, 270EX II & MR-14EX II