I own the 100-400 for two years, mostly used for bird photography with 7D. I have had both dust and moisture problems and finally died a couple of months ago. Now it is back from canon repair after some $300 refurbishing. It is a very versatile lens and you can even do some great macro shots by adding a Hoya +4 macro lens in the front. But... my prime 300mm f/4 IS L has better image quality and is sharper, so now it has become my preferred birding lens specially after adding a Keno 2X TC.
Here is a link to some tests:
I will probably not upgrade to the new 100-400. I would be more interested in an improved version of the EF 300 f/4.
I was shooting hawks (not literally, I was photographing them) today with my 1D Mark IV and 100-400L lens. It was fantastic. What other lens would I have used? I was able to zoom from 100 to 400mm whenever I needed to and on a 1.3x factor, even more. Try zooming out on birds in the sky and as they "swoop" down, you need to bring the focal length in, which can be difficult of course, but not if you practice. Overall I think the lens is pretty good. I'm not sure if it would be easier to rotate vs. push/pull in my situation today or not. Maybe.