I am dying to see some real-world side-by-side comparison shots taken with the original 100-400 compared to the new 100-400 II. By that I mean, some good tripod-mounted shots of the same static target/scene, done at the same time (same lighting) with equal framing and matching (and varied) apertures and ISOs, etc.
When I first got into digital SLR photography I spent a great deal of time (and money) collecting lenses and evaluating them. Then, like so many other photographers, I one day realized that about half the lenses I owned never got used. I had my favorites, and that was pretty much all I used.
One of those favorites is my 100-400. I wasn't too terribly happy with it when I first got it (purchased it used) but, once I sent it to Canon for service, I got back a lens that is a VERY good example of a first generation 100-400. It's the only lens in my bag that requires absolutely NO in-camera micro focus adjustment.
But, as happy as I am with this extremely versatile lens, even on its best day it is not as sharp as my 70-200 f/4 IS (I know, I know, very few zoom lenses are as sharp as the 70-200 f/4 IS). However, I'm holding out hope that the 2nd generation 100-400 will close the gap.
My "gut" tells me that daddy is going to get a new 100-400 sometime next year. But I need those comparison photos to convince me it's worth the investment!
"If a day goes by without my doing something related to photography, it's as though I've neglected something essential to my existence, as though I had forgotten to wake up."
-- Richard Avedon