gentoo is great in that aspect that is's as close to LFS (linux from scratch) as you can get without having to do boring chores of that distribution (get patches, get tarballs, checks deps, unpack, compile, strip, install etc).
i always wanted to use from-source distro. when i heard about gentoo i tried it right away. now i'm feeling pretty uncomfortable using binary distributions.
arch linux comes out as my favorite binary distribution so far. it's pretty simple, and with little bloat. it works like a charm where gentoo is not an option - on my second pc, which is celeron 366mhz with 256mb of 133mhz sdram :]
one reason for using gentoo is that it updates its packages more often, even those more "obscure" for typical desktop user - i particulary care for fdm, elinks, mutt, ffmpeg, mplayer and perl related packages. most popular distros update them usually when new release of the distro is out.
second reason is what makes gentoo stand out - i can strip unnecessary features from packages (e.g. build a system with no gnome dependencies, and with no gnome packages as the result). binary distros cannot provide that, and i particulary hate ubuntu in that regard, because it pulls tons of useless (from my point of view) stuff along with the packages i need.