This little informal report is for Johnathan and Ray who helped, cautioned and advised me with my decision to move from the 70-200L IS to the 100-400 for wildlife with my new 20D.
Where Michael reported and Johnathan cautioned "noticably soft" at 4.5, I have found the lens unusable. At f/5.6 usable. At f/8 it is very good and at f/11 it is execellant! Maybe I was spoiled by the 70-200 resolution but I missed so many shots, good resolution with no pictures wasn't working.
Working with 160 - 640 on the 20D shooting small birds etc. is proving a true joy. Getting to f/8 is not a problem as I can handhold 640mm at ISO 400-800 at 1/125 -1/180 using IS.
I like my new digital rig and must agree with Johnathan that "film is dead". My first blow up went to 12.5 X 18 (cropped) on Pictorico high gloss and I sold 2 copies. The resolution was very good and I see no problem, which was my major concern with digital. it looks like 16X20-22 will hold up quite well with a little res up.
If you shoot wildlife I can heartily recommend this lens and camera combination. It's light, fast and clean.