I purchased this lens primarily for hand-held bird photography for use on my Canon 40D DSLR. The lens is typical of Canon's L-series "white" family of professional level telephoto lenses, having very high standards of construction and superb optical performance, even with a 1.4X teleconverter, when conditions allow.
This lens is known as the "overlooked" Canon telephoto, since it does not garner the glamor of its cousins, the really long, heavy, faster, and vastly more costly professional telephoto lenses. It is also known as the "toy lens" by bird photographers who mostly use those really Big Berthas for long-distance close-ups with blurred-out backgrounds.
I selected it for the high image quality wide-open, relatively light weight, and shorter physical length, allowing both hand-held and tripod mounted use, as well as its modest cost (compared to all high quality alternatives). The image quality is extremely high, even wide open at f/5.6, although it does improve slightly stopped down a notch or two. The image contrast, flare resistance, and color saturation are also excellent for a telephoto lens of its "older" fixed focal length optical design.
It doesn't have image stabilization (IS), which maintains the reasonable price, but I don't consider that much of a handicap using Canon's DSLR's, which allow low-noise high ISO settings for higher shutter speeds under decent lighting conditions. Braced against a tree, fence post, etc., and especially in a sitting position with arms resting on knees, for example, the lack of IS is simply not an issue. The focus is extremely fast and accurate with the 40D's sensitive all X-type focus points, and the quickly removable tripod mount is of an excellent design that all tripod mounts should have.
As a bonus, on the 40D, the lens has the equivalent field of view of a 640mm f/5.6 telephoto lens! When a high quality teleconverter can be used, this becomes almost a 900mm f/8 telephoto (actually 896mm)! Not to exaggerate the point too much, but that's starting to get into the Big Bertha focal length range, under the right lighting conditions. Other pluses include the handy built-in sliding lens hood and the very high quality included lens case, which are extra cost items for Canon's non-L lenses.
The 400mm and 500mm zooms all seem to suffer from image softness at their maximum focal lengths, which I would be using 90% of the time anyway, so I elected to simply get the highest quality fixed 400mm I could find for a reasonable price. I also decided that I could live with 1 f-stop less than the much more expensive f/4's, since this lens can be shot wide-open at the same high quality as the f/4's stopped down - thus resulting in f/5.6 anyway.
This reasoning is even more appropriate when including the slightly less expensive, still high-quality, third party lenses, such as Tamron, Tokina, Sigma, etc. I reasoned that since I plan to use this lens for the rest of my life, why compromise in image quality for a few percent lower initial cost? I'm not slamming third party lenses in general by any means (I own a few), just in this case of comparing available alternatives (including other Canon's) to the selection of this particular Canon 400mm and its intended use.
I was blessed with several excellent bald eagle shots on what I had expected to be just an initial "practice with the new lens" outing. Because of this excellent lens, it turned out to be a very productive shoot. I would include a thumbnail photo of one of these shots, except I can't seem to be able to "paste" a small image into Amazon's review window.
For one that is willing to accept and operate within the parameters of this lens, that photographer will be handsomely rewarded by Canon's 400mm f/5.6L telephoto lens. In my case, it is exactly what I was looking for and it is exceeding all my expectations.