Well I've just come back from Canon's road show at Calumet in Glasgow.
They had all the super tellys (but not the 400 DO) on show, and it was very interesting to do some comparisons.
The 200-400 +TC is certainly a very sharp and superb performing lens, and takes a further 1.4 converter (On selected bodies).
AF is excellent, and the newish IS also great, but personally I'd hate to have to hand hold this lens for any length of time....in fact I couldn't!!
So we come back to the alternatives, and really if you use these focal lengths in their entirety, of course no single lens covers the spectrum.
If you were to spend a lot of time travelling and space was at a premium, then this is an ideal lens, however there is no doubt that it is extremely expensive and very heavy, and personally I'd go down a different route, with a combination of bodies and lenses...and use a 400DO!!..it's just as sharp.
Be interesting to see how the sales hold up after the initial interest!
The other super teles are well known, nice to feel the weight reduction esp in the 400 2.8, and certainly when I compared my mk1 300 2.8 plus 2x TC mk3 and the mk2 300 2.8 plus 2x, the major difference I felt ,apart from the slight weight loss, was the new IS although when I compare the sharpness of the two files, really I couldn't tell the difference.
I think the 1 Dx af system has markedly improved the af on this combo, and really I couldn't justify upgrading to the mk 2 from the mk1 300.
Bit rambling when I read this, however the main points come over.
George.