I have used Source Mage for quite a while but I've come to the conclusion that Gentoo is better in about every way from the init-system to installing software (though "casting spells does sound cool). What annoyed me the most were the endless questions it asks you before casting where Gentoo does this with just a few USE flags. Source Mage allows for a little bit more fine grained customization this way than Gentoo but I don't think it's worth it overall.
After you got one of the many dozen questions wrong for the third time on spells like mplayer you will probably agree.
Though, if you're looking for a source based distribution that is not Gentoo you might like it.