I can answer with direct experience. I started my wildlife photography with a Sigma 400mm f5.6 lens back in 1989. I exchanged it for a Canon 400mm f5.6L. Being what I can only now describe as "indoctrinated by the views of other photographers" I felt I just HAD to get an f2.8L - when I exhanged the f5.6L for the f2.8L I found the impractibility of travelling with such a large heavy lens completely cancelled out any perceived advantages. When I used it wide open at f2.8 I found that the depth of field was so limited I had to shut down to f4 or f5.6 to get an acceptable d. of f. Eventually after a couple of trips to Kenya and India with the 400mm f2.8L I exchanged it for a 400mm f4 DO lens at a cost of several thousands of £££'s. My hopes that I'd found my dream lens were dashed however when I discovered after buying it that it was still extremely heavy, (light? - don't make me laugh)! unacceptably bulky, and the lens hood, (as was the f2.8's), resembled a waste-paper bin. I then exchanged it for a 100-400mm f4.5-5.6L zoom lens, this was smaller. lighter, and more versatile. I have no issues as to the quality of the images any of these lenses provide, other than my original Sigma which wasn't so good, - I will be keeping my 100-400 with a view to exchanging it for a new improved version as soon as one appears. I'd also be interested in a new 400mm f5.6L with IS if they ever brought one out. I've also bought the recent 70-300mm f4-5.6L which is a cracking lens and in my view a new 100-400mm should simply be a slightly larger version. In the days when I was using the Sigma, 400mm f5.6L and f2.8L lenses, I was using a film camera at 100ISO. Now with a 400mm focal length on a 7D body I'm getting the equivalent of 640mm on auto ISO which at say, 400 ISO, is indistinguishable in image quality, taking into account IS which the other lenses didn't have. To anyone thinking of the 400mm DO lens I'd say, wait and see what Canon does this year before spending so much on an f4 lens, which, however attractive it might sound, I'd suggest most people don't really need. They think they do!