I decided to rent the 400mm DO II before buying on. It arrived this morning and I was off to the Gilbert Riparian Preserve for some bird hunting. My main walk around lens was a 400mm 5.6 because I like maximum mobility and hand holding. I owned the DO I a few years back and was not overly impressed with it.
I will be doing some direct comparison shots between the DO and the 5.6 in the next few days but after viewing today's results my initial impression of the new DO are very good. It appears to have better contrast and color than the 5.6 and is plenty sharp for me. The bokeh seems smoother than the 5.6. It is easy enough to carry and hand hold and while the lens collar doesn't come off the foot does (4 screws) and I will be removing it when I buy my own because I don't need it and it will save a little weight.
The MFD seems better than the spec vs the 5.6 but I will confirm this with a real test.The shots I am posting were done with my usual regimen which is some cropping and a bit of sharpening.
The raw files are available here for download and examination.
Canon 7DII 400mm DO II