The Mythic is probably one of our groups favorite pieces in the CC. A decent amount of material (maybe half?) we had already been using in our campaigns for years, we just had to translate them into more mainstream RM (tone a few things down, kind of 'standardize' some things that were home-brew, etc), but it was one of the new and more original bits, a little inspiration from the Mage Hunter, but with a religious twist. We'd been creating spell lists based on RM2 lists, as well as some of our own, and swapping out spell lists on professions like the Ranger, Cleric and Chaotic long before I even sent the proposal for the CC into ICE. Just as I use the Martial Arts Companion in order to spice up all Pure Arms Users I think the Channeling Companion's Priest concept should point the way to all kinds of customized professions for the GM's that hadn't been doing that already.
I've got a fairly well thought out idea of how the various kinds of magic work in 'my little world' (especially Channeling) but it's not something I would have tried to include in the CC as "the" explanation. The how's and why's of things needed to stay somewhat generic in order to fit in with the most RM users own customized campaigns.