I’ve been using it for years and now I basically can’t live without it. I consider OpenWrt compatibility in all of my router purchases. Currently using a Netgear R7800 and a Belkin RT3200, both are going strong.
It isn’t as widely used because it can be finicky to flash sometimes, and that’s if it’s even compatible in the first place. Even if it works, you may experience a drop in performance unless OpenWrt supports using the routers hardware acceleration features. If there’s no support, OpenWrt basically uses the onboard CPU to do routing and they’re usually not all that powerful.