It's been a few weeks now, and I flat out admit to missing the 400 DO II. The weight of the 500 is a little annoying, which I knew going in, but is the extra stop of light from not using a TC worth the cost/weight? This has been the #1 question since the start, and I knew that it would take a while owning both before I could come to my own conclusions. On the bright side, the 500 is phenomenal in the IQ department (so is the DO).
One intended benefit from having the 500 is that I'm considering using the 100-400 II + 1.4 TC more again, for times when I want to go lighter. The venerable zoom was seeing 0% use when I had the DO II, but now I think it'll get the nod more often. I'm heading to Florida tomorrow, and the 100-400II is in the bag; neither the 400 DO or the 500 would be considered for this trip (birding may occur, but mostly won't), but in the event that it was a bird-centric trip, it'd be nice carrying the DO and not the 500. For instance, I just got the MindShift PhotoCross 13 sling bag, and the DO II can fit, so I could take it if I wanted to. There's the size advantage showing it's benefit.
A very small part of me is weighing (pun?) whether or not to reverse my decision, and return to the DO II. Part of the joy/pain of this hobby, I guess, sometimes it takes a lot of trial and error (and $$$) to figure out exactly what you need in the end.