I ran Gentoo for a long time in the early 2000's. I learned nearly everything I know about Linux machines in general from that experience!
What was interesting about the USE flags was learning that a given package even HAD a particular integration with some other library / package. Realizing that the SQLite3 binary doesn't work the same when you don't have readline support linked to it lead me to understand what readline was as a whole. That happened over and over again for a lot of the "invisible" libraries that are always included on every other Linux system.
Absolutely invaluable learning tool at the right time in my life for sure.
Yup... still remember it! Crazy!
Sad to see this go.