I have backed a lot of KS projects and bought a fair amount of stuff on DriveThru/DMs Guild.
Really a lot.
I haven't used every bit, but I've used 3rd party rules, classes, subclasses, other options, monsters, setting stuff (heck, my main D&D is the Tal'Dorei setting), magic items, ancestries, ideas on separating ancestries and cultures, and other wacky stuff. I've used more 3rd party than I've used official in what I add into my game. Wanna play an Artificer? Step off! Wanna play a Witch (Hidden Oddities: A Witch's Primer is my favorite book)? No prob!
I used a lot and read more.