I photographed a stuffed valley quail to understand how my copies of the Canon EF 400mm f/4 DO IS USM and Canon EF 100-400mm f/4.5-5.6L IS USM lens perform under different aperture and shutter speed settings. I also photographed with a rented Canon EF 400mm f/5.6L USM for comparison.
Disclaimer : I am only testing one copy of each lens type and I am not sure that the copy tested is representative of the population.

**No sharpenning or other alterations were applied to any of the cropped images other than to convert the images from RAW to JPEG using PS CS3. The images were cropped to 700x700 pixels and then cut and pasted side-by-side for easier viewing.**

This is what I learned:
1) The Canon 400mm DO and the other lenses tested (data not shown) required no microadjustment on my 50D camera.
2) I can handhold the 400mm DO lens w/o image stabilization as long as the shutter speed is faster than or equal to 1/350s.
3) The pictures taken with the 400mm DO lens fully open (f4) are very good with a slight improvement when the lens is stopped down.
4) The image quality produced by the 400mm DO, 400mm f5.6 and 100-400mm (at a 400mm focal length) lenses at the same f-stop are virtually identical. It is very difficult to detect differences if any even when looking at the 100% crops. In summary, Canon has some very good choices at the 400mm range and the three copies that I tested should yield very good bird pictures in the hands of any skillful photographer.
5) The 400mm DO lens was the only lens that would autofocus with the Canon EF 1.4x II Extender (Teleconverter) on the 50D camera. To my eyes the TC did not affect image quality.

A recent review of the Canon 300mm f/2.8 can be found here: http://www.pbase.com/juliom/300mm