I've just plugged my Apple TV 4K into my Sony 43XD8305 TV using an AmazonBasics HDMI2.0 1m cable, on wired Ethernet (on 200Mb cable Internet).
Out of the box setup is insanely great, just waved my unlocked iPhone near the AppleTV and I got prompted to enter a pairing code and then my separate iCloud/iTunes account details were set up automatically. Nice.
I got prompted if I want to enable a feature that sounds like it will sync all my of app choices between my two Apple TVs. Nice.
The TV was immediately reporting that the screen mode was 4K. Nice. Then the Apple TV offered me the option to try and enable HDR, which failed and it reverted back to SDR. Not nice. Turns out my stupid TV only supports HDR on two of its three HDMI inputs. Switched it over to another input, went back to the relevant Settings menu to enable HDR again, but it had already auto-negotiated it. Nice.
There's also a menu option to switch from 4:2:0 to 4:2:2, but I don't know if it's going to let me switch back easily if it doesn't work with my cable/TV, and I'm not even sure how useful that is, since AFAIK 4K movies are encoded at 4:2:0. I don't even really understand what the difference is, but one is a bigger number, so when I'm feeling a bit braver, I'll give that a go
Fired up the iTunes Movies app, I'd already checked out the 4K HDR movie list in iTunes on my Mac yesterday, and I saw that the most recent movie purchase I'd made, which supports 4K HDR, was Alien Convenant, so I chose that, and jumped to the scene when they're walking through the corn field. Realised immediately that Ridley Scott's choice of colour grading was so muted and earthy that I would never see anything from HDR, so I popped back out and enabled the screensaver mode, which looks incredible.
Netflix app works just fine with 4K HDR (tried a bit of Narcos). Didn't bother trying YouTube because of the stupid VP9 codec situation, or Amazon's app because it doesn't exist yet.
All in all, seems pretty great. HDMI-CEC works perfectly as it did with the 4th gen Apple TV, but on the whole, TVs are awful rubbish in terms of their features, UX, and documentation. I wish Apple would make an actual TV.
One open question I have, is that some 4K HDR movies in iTunes show badges for 4K and HDR, while others show badges for 4K and Dolby Vision. I don't yet know whether the latter means they support HDR10 *and* Dolby Vision, or just Dolby Vision. My TV only supports HDR10, but since the AppleTV seems to always be outputting 4K HDR10, I'm really not sure how I'm supposed to tell (and honestly, for all the hype about HDR, I actually think it makes a pretty minimal difference unless you're in very optimised watching conditions).
So, there is my quick summary of stuff I've done with it so far