I'd say the D500 is maybe 1-1/2 to 2 stops
** worse than a D4S at mid-range and higher ISOs. I don't have a D5.
On a D500, I have no hesitation of using ISO 25,600 (well, maybe a bit of hesitation) and I'd use ISO 51,200 if I really, really had to. ISO 100,000 might be useable but that will depend on your definition of useable.
At ISO 200,000, everything has to line up just right – good lighting, no fine details, small reproduction size – and/or you are very, very desperate.
The top three ISOs (ISO 400,000, 800,000 and 1.6 million) aren't really useable.
Below ISO 12,800 or so, I don't think the difference between the D500 and D5 is huge. But remember that the D5 costs three times as much.
Problems with the D500 are:
• The focus tracking is missing the "d9" option (to track using 9 focus targets). No idea why it's missing and no idea why it hasn't been fixed through a firmware update. The D500 has been out for over a year.
• The built-in wifi/bluetooth is not at all what you expect it to be. The wifi/bluetooth works only with proprietary Nikon software (which is not very good) and only with cellphones and tablets. For me, the built-in wifi is completely useless. I assume this could be fixed with a major firmware update but I doubt Nikon will do it. By contrast, Canon's 5DmkIV apparently has very good built-in wifi.
• No voice annotations. This, too, could be fixed through firmware but it won't happen.
• No ethernet
• Battery life is adequate and that's with the wifi/bluetooth turned off. With every other Nikon camera I've had, I've been able to get reasonably close to Nikon's estimated number of shots per charge. But with the D500, I get about 60%-70% of Nikon's estimated number of shots per charge. I'm using Nikon's newest Li-ion20 batteries.
I bought the D500 as a temporary camera: I needed a camera last year, Nikon hadn't announced its two new full-frame cameras yet
and the D500 was one-third the cost of a D5.
Also, the D500's 1.5X crop factor means that it was cheaper to buy a D500 to use with my 300 f2.8 than to buy a 400 f2.8 or 500 f4. Think of a D500 as an expensive teleconverter
When on sale at $2300 (and even at $2200), the D500 is a pretty good deal. The battery grip is very overpriced. XQD cards are nice. The tilt-screen does come in handy once in a while. I don't use the touchscreen.
**Added: By definition (and by physics) small-sensor cameras
always have at least a one-stop disadvantage compared to a corresponding full-frame camera. Smaller sensor => smaller pixels.
For example, you cannot fairly compare a D500 at ISO 12,800 with a D5 at 12,800. A fairer comparison might be with the D5 at 25,600 to 40,000.