Got mine installed today - upgrading from a 6700k - and I'm pretty much blown away by it. By my reckoning each core is about 50% faster than the old i7, which I really wasn't expecting given the modest increase in clock speed (was at 4.8GHz previously). By virtue of that and double the cores, 3D rendering performance (a significant thing for me) is way over three times more powerful.
As for gaming, I was hoping for big gains in VR cos I knew my 1080Ti was severely CPU-limited. Am now running Assetto Corsa with 200% supersampling and it doesn't skip a beat at 90fps, it's a huge difference. While running a few benchmarks before upgrading I was surprised to see I was also CPU-limited in Assassin's Creed Odyssey @2560x1440. GPU usage never went above 70%. Needless to say that's not the case anymore, Min/Avg/Max FPS went from 14/58/81 to 43/79/137. Considering I thought I had a decent gaming CPU previously, that's pretty astonishing.
I get that much of the above would be pretty similar with an 8700k or many other recent CPUs, but from a personal point of view I know the two extra cores will prove useful.
Haven't done much in the way of overclocking yet. The Asus (Hero) AI optimistically tried 5.3 but that wasn't stable. I am however now at 5.2 (still with the auto-overclocking) and that seems fine. Max temp I've seen so far is 70C, which is probably because I've got 600mm of radiators dedicated to the CPU since my GPU isn't in the loop any more.
Feeling quite pleased this evening.