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Stu, my recommendation to you stands. If you want top notch performance and ease of use, 500 II is a great choice. Also, the 500 MkI is still a great lens, just a bit on the heavy side.
If you want the ultimate supertelephoto lens at the expense of some additional weight and bulk, then 400 f/2.8 IS MkII is it. The lens also makes fine 560mm f/4 and very good 800mm f/5.6 with the MkIII TCs.
Peter, I know you're making sense, but the 200-400 just tugs on my little boy sense of fun My Nikon shooting mate with a D4 and 500 VR also keep telling me the 500 ii is my best option. Although a 400 ii on the 1D4 might be more useful than a 500 ii on a 1DX...Come April, if the finances are ready then I'll make a firm decision. Judging by the overwhelming lack of enthusiasm I've seen on various blogs and fourms, if Canon do release the 200-400 at very high price it may well be a big white flop. I'm sure we'll know more within the next couple of months