Yes, IMHO. I often use the 1.4x and the image quality does not suffer and in some cases, it's better. Adding the 1.4x turns the 100-400II into a nice macro lens and a super zoom at the other end of the spectrum. Limitation is not a good lens in low light. I have upgraded over the years to some excellent L glass (up to the 500mm II) and the 100-400mm is currently the one lens I grab for a "do it all" lens.
