Just returned from a week in south Florida and the Everglades. Visited excellent bird sites at Loxahatchee, Tigertail Beach on Marco Island, Shark Valley, Anhinga trail in Everglades Park, and others... Used my 400 with some fill flash, with some great results. Sharper than my 70 to 200 2.8 for most situations. Looking forward to getting down there again!
Some of the birds were very close, about 10 feet away, others about 30. With the 5Dmk2 full frame I can crop substantially and keep the quality high. I've got about 4000 other pictures last week of the full bodied birds and alligators, but I seem to enjoy getting close to their heads, especially if it's still sharp. Most of the sites I went to have birds and gators that aren't spooked as much by humans, so it wasn't too hard getting close. I also use Aperture fyi... Al.
Here are a couple of Loon pictures shot with my 7D and the 400 f/5.6. A very sharp - and relatively inexpensive - 400mm option; you just have to have it on a tripod, or have a high shutter speed, to avoid camera shake and the lens really shines.
With two young kids, I get very little time to play these days. However, a trip to the local zoo today allowed me to take my EF 400mm f/5.6L along with the EOS M and adapter. I very rarely get a chance to use this lens nowadays. The combination isn't ideal because with long lenses you really want to get the camera up to eye level and brace yourself in order to keep it steady. Holding this combo at arms' length is awkward and needs some kind of support.
I still love this lens, even though I may consider trading it in for the new 100-400 (I never liked the original 100-400). I also love it because the lens it replaced, an EF 300mm f/4L IS, was an absolute dog and gave me all sorts of problems. The 400 has been completely reliable and the keeper rate is excellent. With the 300 the keeper rate was about 2%. It must have had some kind of a problem, even though the Canon Singapore service centre calibrated it and told me everything was OK.
At f/5.6 the depth of field is pretty shallow with the 400. I was trying to keep the tiger's eyes in focus, but his nose is out of focus. Bridge tells me that the subject distance was 6.5m. I have a tendency with this lens to put the camera into AV mode and just keep it open at the maximum aperture.