Builds everything from source and most packages have flags ("USE flags") that control in which way the source packages are built (e.g. remove unneeded package dependencies). The other potential benefit is to compile with better optimization than generic packages.

Also rolling release/upgrade instead of the usual scheduled release.

Mainly that you can tweak the compiler options to your liking, and have the full userspace built with it. For example, you can target your exact CPU with all its particular extensions. There are also hundreds of compile time feature flags, so you can omit functionality you don't need.

I've used it for a long time before switching to Arch and then Ubuntu. When using Gentoo I used to know more about the different subsystems running - it was a function of seeing what needs to recompile when you update and also having to sort out issues when certain hardware stopped working.

I stopped using Gentoo and Arch because it was just easier to be on the same OS as other devs and it didn't feel like I lost much switching to Ubuntu. Every now and again I'm tempted to go back to Arch, but honestly, there are other things I'd like to play with rather than my OS

The advantage of Gentoo Linux is you get to see lots of compiler output scrolling past instead of being able to do real work on it, so you look like a l33t h4xx0r.

The disadvantage is that everything is built from source, so it takes forever to install anything and it's very fragile, and it's a rolling release distro so you're installing stuff every single day and it's very fragile.

Run Gentoo if you have lots of time to spare. I did when I was in college.