Old timer here, I did a mostly full PC upgrade after many years, replacing a 4790K/Z97 setup with a 10700K/Z490 one. I wanted the modern platform (USB-C, onboard wi-fi, etc.) more than speed but used that an excuse to satisfy the upgrade itch. 
I almost got a 3900X/X570 instead but mainly use the desktop for games and emulators, and do work and other things on a separate laptop. To be honest the 4790K still held up quite well, but I can see some improvement in heavy action situations in a few games, and I want to do VR and replace my OLED TV with a 120hz version in the future. I kept my old video card for now since that has been upgraded much more frequently than the rest of the stuff, but might replace that when the new stuff comes out later this year. It's impossible to find the Comet Lakes at MSRP anywhere, but Microcenter has some decent combo deals on them. I might do a mild OC on the CPU but it might not be worth it for a few hundred mhz. I miss the old days when you could often get 50% OCs on top end CPUs, which I did on the last 6 or 7 I had before the 4790K.
I got a Corsair 500D RGB SE case, which replaces a big Cooler Master Stacker that I had been using for over 15 years across many builds. It's kind of overpriced but feels high quality, and I like the whole ecosystem of RGB and fan controllers it comes with. The last time I looked into cases and fans, none of this stuff or things like cable management, shrouds, etc. existed. The dark glass panels look great with the lights shining through them, subtle and classy, just like high end audio equipment. AIOs seem to be popular these days but I decided to stick to air for its simplicity and reliability, especially since the video card would still be on air either way. The only thing I miss are the casters the Stacker had, but they seem to be very rare on cases these days. The cable management takes a lot of effort to set up, but is rewarding once you get it right. The cable cover the case comes with doesn't fit on if you have too many cables, but is not really needed.
I put all my old stuff on ebay like I've always done. It's surprising how the 4790K still holds its value, for a 7 year old CPU.
The new rig and the old one:
I almost got a 3900X/X570 instead but mainly use the desktop for games and emulators, and do work and other things on a separate laptop. To be honest the 4790K still held up quite well, but I can see some improvement in heavy action situations in a few games, and I want to do VR and replace my OLED TV with a 120hz version in the future. I kept my old video card for now since that has been upgraded much more frequently than the rest of the stuff, but might replace that when the new stuff comes out later this year. It's impossible to find the Comet Lakes at MSRP anywhere, but Microcenter has some decent combo deals on them. I might do a mild OC on the CPU but it might not be worth it for a few hundred mhz. I miss the old days when you could often get 50% OCs on top end CPUs, which I did on the last 6 or 7 I had before the 4790K.I got a Corsair 500D RGB SE case, which replaces a big Cooler Master Stacker that I had been using for over 15 years across many builds. It's kind of overpriced but feels high quality, and I like the whole ecosystem of RGB and fan controllers it comes with. The last time I looked into cases and fans, none of this stuff or things like cable management, shrouds, etc. existed. The dark glass panels look great with the lights shining through them, subtle and classy, just like high end audio equipment.
AIOs seem to be popular these days but I decided to stick to air for its simplicity and reliability, especially since the video card would still be on air either way. The only thing I miss are the casters the Stacker had, but they seem to be very rare on cases these days. The cable management takes a lot of effort to set up, but is rewarding once you get it right. The cable cover the case comes with doesn't fit on if you have too many cables, but is not really needed.I put all my old stuff on ebay like I've always done. It's surprising how the 4790K still holds its value, for a 7 year old CPU.
The new rig and the old one:
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