So I'm a proper devil now
Some observations -
You get unlimited power word, dominate and HFR casts. Which is kind of nice if you built for it. I, however, did not as I was going for more of a martial inquisitor build.
You also get Fire immunity, which ironically means that Hellknight does not synergize that well with devil.
It also doesn't synergize that well with Infernal Bloodline Sorcerers as they get most of what that mythic path offers (and more. Power of the Pit grants both fire AND poison immunity), which is kind of weird. I guess it does encourage some multiclassing as you get back the abilities you "lose" but still, kind of awkward.
In fact, Devil seems to be better for support characters or casters, as you get a lot of nice support abilities and spells but nothing that really increases your BAB other than Aeon Bane
That Hell Seal that instantly kills an allied target if their health drops low and deals damage to enemies seems to work on summons and dominated enemies, which makes it a lot more viable.
You still get Gold Golem Summons at Rank 9 as devil, which is weird to me. You'd think you can summon something more diabolical.
You do not get a Devil Mythic Spellbook; your spellbook effectively stops at level 5 spells for Aeon, which is lame.
In short, kind of cool but still feels half-baked. I don't feel weaker, which is what I was worried about, but the fact that its a late game path that doesn't entirely synergize with all builds (you absolutely need Fire Ascendency and Spell pen to make the most of Devil) feels awkward and that you still get some Aeon path abilities in the form of Golems feels phoned in. You don't even lose the Aeon version of Bound of Possibility, which is great because that enchantment is amazing, but still, kind of lame for a DEVIL mythic path.
Apparently the best designed paths are Angel and Demon, which if true indicates that they worked on those the longest and most other paths afterwards came later, with the late game paths being mostly afterthoughts that they just squeezed in there because they thought it would be cool.