Went from an i7-8700K with an Asus Strix 1070ti on a standard ATX build (NZXT S340 Elite) to the i9-9900K with an EVGA Hybrid GTX 1080Ti FTW3 on a MicroATX build.
I use the machine mainly for gaming (Fortnite, Black Ops 4, Apex Legends, Overwatch) and also stream using OBS Studio on Twitch. On the side, I use this as my project computer for creative content with Adobe suites including photo and video editing. It is my complete workstation and workhorse. I originally came from all Apple products, and still use all their mobile and iOS devices when I travel.
Always wanted a microATX build after completing my full size build and realizing there was absolutely no reason to have a full ATX PC. Everything fits in my NZXT H400i very well and air flows very well. My GPU averages 40-50c under load using a push/pull configuration for the 1080Ti radiator. i9-9900K is overclocked to 5GHz on all cores with an offset of -2 at a 1.345v. Temps under 100% load peak at about 83-86c.
Only ran into some issues with the CAM software as it's pretty garbage. Also wished the H400i supported more HDD drive as it only fits 1x 3.5" but I will attempt to either squeeze another one or just build a NAS drive.