I shoot wildlife (mostly birds) and use a 7D with a 300 f4, often with a 1.4 extender.
This past summer I became interested in possibly purchasing a 400 5.6 thinking it might be a better way to go then using the 300 with the extender.
Before buying however I had the opportunity to try out the 400 first. I was able to test two different 400 5.6 lenses. One had some use, the other was new right out of the box. I tested them on two different 7D bodies, and one 50D body.
I tested them both in the field on bird action, and on stationary objects until controlled conditions.
In virtually every instance my results were that the 300 f4 both alone and with the 1.4 extender, produced a sharper, more detailed image then either 400 5.6 tested.
It got to the point where looking at images taken with a 300 plus extender side by side with images taken with the 400, without knowing which was which, I could immediately pick out the ones taken with the 400 as they appeared softer.
In addition, although the 400 may have been just a bit quicker in the AF, vs, the 300 with 1.4, it was barely noticable.
Now I know a lot of people use a 400 5.6 for BIF and get fine results. But these were my experiences.
I passed on purchasing the 400 but may reconsider down the line should Canon ever introduce a new version with updated optics and focusing.
As far as the 100-400 goes, I'm not a huge fan of super zooms, and don't care for the push/pull operation of this lens. I do prefer a prime, but know some other photogrpahers that do fairly well with the zoom. Although I hear some say they have actually missed action shots because they were busy zooming when they should have been concentrating on getting their pictures.
I also know at least three photographers, that either plan to or already did sell their 100-400 and switch over to a prime.