Upload & Sell: Off
If you have to stop down to f5.6, then there would be no point in buying the DO.
But I regularly do use my 400 DO wide open. When I do stop down, it isn't for image quality improvement, but it is because I want to increase depth of field. An f4 lens, at least, gives you the option of shooting at f4 when the situation can benefit from higher shutter speed, more depth of field, etc. No matter how hard I try, I can't make an f5.6 lens work at f4.
I have not seen anything stating the 400 DO focuses faster than the 400 f5.6. So if someone is shooting with the DO at f5.6 all the time, please tell me what the advantage is in having the DO.
Since I sold my 400/5.6 before I got the DO, I can't make a direct comparison, but I do know that the DO jumps into focus about as fast as any lens that I've used, and, as pointed out by Sneakyracer, the IS is a big plus for the DO.