I'm thinking about replacing my 100-400L with this lens as I just bought a 300 mm f/4 prime (non IS) which I can use with a tc 1.4 for the long end. The new generation IS, the low weight make this new lens a far more versastile lens I think.Here
is one more test that adds to the rumour... " a hidden L lens" it says.
Having just bought one I loved this line from the review linked above:
"Compared to true
L grade lenses the contrast level seems to be a tad lower at 300mm. Nonetheless this is truly baffling for such an affordable lens."
I haven't had a lot of time to play with it yet but I took quite a few hand held shots of my Beta (fish) in poor light for lens speed and was quite surprised with the results.. Very nice for the little experiance I have with it so far.