I'll have to read up more on the 200-400, but it seems to share the extreme optical quality of the recent big white lenses. What it seems to have over the 1D X, 300/2.8 II, and a 2xIII is, of course, the internal TC. In Rouse's article he talks about the potential advantages of the built in TC, namely he implies that the AF speed may not suffer as much as non-built in TCs, the speed at which you add the TC inline, and fewer opportunities for dust to enter the system. He has some sort of quote about less time cleaning your sensor and more time shooting. It seems to be a great lens. I'd say it favors those whose distance to subjects will vary quickly or frequently and you have reasonable lighting. I agree, this could be the ultimate safari lens. But my guess is that it will also become prominent for outdoor sport photographers as their subject distance changes both frequently and rapidly. For me, when I've saved enough for a big white (whenever that is), it will get some consideration, but my guess is that 560 mm isn't as long as I will want. I'd have to own a higher MP camera to consider it.