Gwilliam:
There are other source-based distributions.
True, and I haven't tried them. The big thing for me about a source distribution per se was just being able to "ebuild ($equery w ... unpack" and then be able to poke around, attempt to fix bugs in the packages I was running. If other source based distributions make it this easy, that's great.
Using Gentoo taught me much more about Gentoo (Portage, slots, overlays, emerge, baselayout, Gentoo's meta-packages, sets, USE flags, et ceterarn quite so much regarding things that are unique to that particular distribution.
I would argue that this
insn't unique to Gentoo. Studying portage taught me the mechanics of buiding a distribution in the first place, lead me to autoconf, what to expect from a package manager, how dependencies and multiple versions are handled. I class this as "under the hood" too. Because I had to make my own choices, I learned much more. When it came around to having to maintain a CentOS or Redhat system, or install a package from source somewhere without a package manager, I found I picked it up with little effort because I already understood the concepts. Learning this stuff means you learn about distributions in general, not just about Gentoo. Once again, this might apply to other source distributions too, I don't know.