Arthur Morris is the doyen of bird photographers. For many years, the 400 f/5.6 was his "favourite toy" lens. Here is a typical quote from his blog:http://www.birdsasart.com/faq_1-4isor4f56.html
Which is a better lens, the Canon EF 100-400mm f/4.5-5.6L IS zoom lens, or the 400mm f/5.6L lens?
I'm asked this question almost every day (sometimes more than once) -- either in person, on-line, or by phone. I always answer it with a question, "Do you want to use the lens primarily for bird photography?"
If yes, then the straight 400 is clearly the lens for you. It is the world�s best flight lens. It is lighter than the 100-400 zoom. It costs less. It will give sharper results with the 1.4X teleconverter than the 1-4 zoom. The speed of initial focus acquisition is unmatched. When used with an EOS 3 body and mounted on a fairly sturdy tripod, you'll have a great starter outfit for bird photography -- a sharp 560mm f/8 lens with functioning autofocus.
But, a few years later comes this confession:http://www.birdsasart.com/b13.html
Confession #1: Though I still consider it the best lens in the world for photographing birds in flight (see FAQs on web site for details), I no longer carry my beloved "toy lens"--the Canon 400mm f/5.6 L--on my shoulder as my auxiliary intermediate telephoto. It has been replaced by the Canon 100-400mm f/4.5-5.6L Image Stabilizer zoom lens. I have been using the 1-4 more and more every day and--contrary to some reports from other users--have been making razor sharp images at all focal lengths with wide open to moderately stopped down apertures. In addition, I have it used it wide open, handheld with the 1.4X tele-converter with excellent results (with static subjects) at 560mm. For bird photography, the versatility of this lens is unmatched; I find myself making images that I would never even have thought of before--especially of groups of birds in their surroundings. And though it is heavier than the 400 f/5.6L, it is also a superb flight lens. At Bosque Del Apache NWR late this fall, I used the 1-4 on a tripod before sunrise for "bird-scapes" and then again almost exclusively for the spectacular blast-offs. I only wish that the zoom were a bit smoother. I am even considering selling one of my 400 f/5.6s; I never ever thought that I'd say that when the 1-4 first came out.....
In my opinion, it is simply preposterous that Canon still makes a 400mm lens without IS. When I want a 400, I put the 1.4xTC on my 300mm f/2.8 II. It's expensive but not too heavy, and four stops of IS make all the difference in use. Or, I take the 100-400mm when weight and size are concerns or I need a zoom. What's the pint of carrying a lightweight lens if you have also to carry a tripod to get the best out of it?