I had the chance to handle both the D5 and D500 tonight at a Nikon presentation at the
local camera store. Both are pretty darn nice. We weren't allowed to use our own memory cards, but we could take shots and look at the results on the LCD screens. The D500 at ISO 51,200 (the fastest native speed) looked pretty grainy, but I have nothing to compare it to. And of course they were pre-production models, so things might change a little when they're finally released in a few months.
One feature that I was worried about was the touch-screen feature on the LCD screen. I have problems with my nose pressing the buttons on the back of my camera, turning on the LCD screen when I'm trying to compose a shot. The last thing I need is for the LCD screen to react to any body part that touches it inadvertently. In reality, the touch function only works in playback or live-view mode, or when entering text data on the menu. I believe it can be disabled altogether as well.
I heard someone ask about the Q setting on the mode dial. It stands for Quiet mode, which will be useful when we're all shooting Nemo at Pixelmania. (Don't want to annoy guests with the constant shutter noise, do we?)
My biggest complaint is the lack of accessory compatibilty. My D300 and D700 can share batteries, battery grip, memory cards and cable release. The D500 has a different grip, different battery and different memory card. So If I get one, everything I own except lenses becomes obsolete and must be replaced. (Not that I plan on dumping my D700 anytime soon.)
Still, it's tempting. I'll certainly look at prices and probably rent one later this year to try before I buy.