Keithr and jrista, just curious, are you guys not bothered by the donut bokeh? I've never shot with a DO lens and don't do enouggh birding to know how critical that is, so I'm interested in your take on that.
I don't believe the 400 DO has "donut bokeh". Thats usually something you get with catadioptric lenses, and is caused by secondary mirror obstruction. The 400 DO is NOT a catadioptric lens, its a standard type lens with a couple specially ground glass elements in the front element group....very different things. From what I've seen, the bokeh of the 400/4 DO is better than the 100-400 @ 400. Sharpness is also better, from what I've been able to tell by digging around the net for comparison shots. People complain about contrast, but it must be an incredibly minor effect, as I've never been able to notice any difference in contrast myself. A good writeup about the lens with some sample shots comparing the 400 DO with the 300/2.8 can be found at Luminous Landscape: http://www.luminous-landscape.com/reviews/lenses/400-do.shtml.
I think there is a whole subculture mythology that has sprung up around the 400 DO, and its given this lens a really bad name based on a bunch of assumption and unfounded speculation. The diffraction grating will take a toll on resolution
...but it is a MINUSCULE toll...not something that is going to matter unless you are enlarging your shots 3 fold or more, and even then, thats really pushing it for ANY lens. That assumes your camera is capable of capturing all the resolution the lens has to offer in the first place...if its not, any complaints about the 400 DO not offering as much resolution as a non-DO counterpart are pointless on their face. For everything else, from all the real-world reviews from actual photographers (photographers, not reviewers), the IQ seems to be SUPERB. Its really sad that people think its such a terrible lens...IMO, its a genius lens, offering pretty awesome optics and a wide aperture in an amazingly handy, useful size that won't tire you out through a day of tracking birds or wildlife as much as a normal 400/4 lens would.