Although this is "only" an F4.0, the main advantage is weight!
I use this lens with a 1DX and the weight balance is perfect. I can shoot handhold without any problems for an afternoon. The IS is unbelievable, I can shoot stationary objects, e.g bird sitting on a branch at 1/60 handheld and still get sharp results.
A lot of folks do not like the DO lenses, the "onion" effect in backlit photos is apparently a problem.
I have shot over 4000 pics in the last month, and indeed it happens. However, it is VERY minor.
All in all, I have doubted a long time to buy this lens as I wanted the 400 F2.8 II.
The weight of the 400 F2.8 II is just too much for me, I can only use it as a tripod lens and not as a walk around lens.
The 400 DO II is just slightly bigger then 70-200 F2.8 II and about same weight, which means that I can take it on a hiking trip in he hills without too much problems.
For me, this lens is ideal, I have the reach, the reduced weight, perfect IS, and very fast autofocus.
Uhh, I did not mention IQ as that is a given as far as I am concerned.
Extenders 1.4, no problems, focus is slightly slower, IQ is still excellent, The 2.0 is a different story. Focus is dirt slow and IQ takes a hit.
I used this lens on 7D mark 1, and the body can not keep up. Focus is slow and inaccurate. AFMA according to Reical V2 was +2, so no problem there.
I made the right choice, I just love this lens, it is just the $$$$
Hope this gives a bit of background info.
Ben