PMR's rationalization seems perfectly reasonable. I do like that he points out the 400 has an eye candy look that is special, which is the number one reason I covet one. My own rationalization and wavering comes down to being conflicted about wanting that eye candy, but also the versatility of a zoom, which to answer the question, I do use the 200-600 for. I am at 300 and 400 a lot with the 200-600 (not so much 200, and 600), esp. since I am photographing youth sports that ends up being contested in the midfield for the bulk of the time. I am not sensitive to weight, since I am stationary, and I will never use TCs like a zoom, or even a 2 body set up, realistically. Whatever is on my camera is what I am shooting with for the day (which is why I love the 50-150GM for everything else).