Haven't had a real go of my D500 but early tests in my lounge room would indicate the high ISO abilities are actually as good as they say. I haven't done a side by side comparison yet but just looking at the images I think it is superior to my D810 at higher ISO.
All photos with Tamron 85mm f1.8 VC. Just using the SOOC jpg's here, which are actually quite good.
ISO 4000 by Crew One Photography, on Flickr
ISO 12800 by Crew One Photography, on Flickr
ISO 32000 by Crew One Photography, on Flickr
Edit: Uploaded them to Flickr for better quality