I bought the new 300 mm F/2.8 II lens and do not regret it. It is small light and very sharp. Handles T.C.'s very good with very good AF. So to replace it, the new 400mm II DO will have to outperform it.
Anyway, the first test to come out that I saw was from Canon Rumors. This guy said that after testing the new 400mm DO II, it was sharper than the older 300mm F/2.8 II lens @ F/2.8. At that point I was ready to pull the trigger.
Just out of curiosity I googled it. And came up with a hit. SLRgear had tested a copy. Oddly enough they came up with a different finding. The new 400mm DO II while being a super lens. Was not as sharp as there copy of the 300mm F/2.8 II @ F/2.8?
Is there that much difference between different lenses? Is one testing method better than the other? Who do you believe!
I personally like SLRgear the best. It seems to be impartial and very forthcoming with there results. And they test ALLOT of different lenses. Much much better than Canon rumors IMO.
This brings me back to what do I do? Personally I am waiting on a few field test reviews with the lens.
But, I very much doubt I will change at this point.