I think it's a wonderful lens. My only complains are:
1) Wish it was lighter (But that is said about any long focal length lens.)
2) Could be sharper. It is very good, but not great. Of course, for the amount of money I'd have to pay for "great", I think I'll stick with "very good". (NOTE: From what I understand, the 400mm DO lens is the same quality as the 400 @400. So that isn't an option! And I want to gain a stop when I upgrade, so I won't use the 400 f5.6.)
3) Quality control. I got a good one, but I've read about others who's pictures a much softer. It happens often enough, and from people that I trust that I believe it's quality control and not just bad technique.
4) Cost. $1,499 USD is a lot of money.
5) The push/pull zoom is annoying, but if you get used to using the locking clutch you get used to it (i.e. it removes zoom creep.)
Art Thorn,
I wanted to double check. Are you blaming the 100-400 for "autofocus was less than perfect with birds in shubbery". The only way this could happen is if the 100-400 caused less contrast in the image that was used by the AF system. I guess that is possible. Are you saying that compared to a lens with the same USM motor system in it, it focuses slower or less precise? (note, you can't tell the speed of the lens just by having USM, I believe there are 3 or 4 different USM systems still in production.) That would be very interesting if it were true! What lens(es) are you comparing it too?
Eric