I just purchased this lens last week (Feb. 2004). It is impressively sharp (with my 10D). It performs best in good light, but with a tripod and remote switch, I can obtain excellent low light photos on slow moving or stationary subjects.
What has impressed me most is it's performance with the Canon 2x II extender. That extender is a disaster with my 70-200mm 2.8L IS, but with the 400mm 5.6L, the results are excellent. Yes, widest aperature is only f11 and you lose AF (quick manual focus takes some practice) with the extender, but the 800mm photos remain surprisingly sharp for the money. You have to work harder because of the aperture "limitation", but I enjoy the challenge. So with this lens and the extender I have a affordable 800mm (albeit slow). I'm suprised others haven't discovered this combination.
Overall, I'm impressed with the 400/5.6L and feel it is a bargain, especially combined with the 2x extender. I would recommend it without reservation.
TL, March 1, 2004