I really like the Mythic rules, and I dont run Paizo APs.
I even backed Legendary games KS and got the 3 Mythic books (Players, Spells, and Monsters).
1. When you are playing low level, your first Mythic tier increases our survivability a bunch.
2. Being that they are "quest based", the reward for completing an important section of the story in in Mythic Tier advancement. Now I know some people also do xp that way (mile-stoning), but to me this makes more narrative sense than just adding up an xp amount. (I am that way in terms of xp as well).
3. Yes, it makes character s more powerful. Sure, my 13th Level (Tier 2) Mythic Psion can do 36d6 damage using the Mythic form of Disintegration, but I spent my Mythic Feat and both Path Abilities to be able to do that. And I still:
a. Have to hit
b. They get a save
c. Beat Power/Magic Resistance.
It also costs power points and a Mythic Use.
Needless to say, i have only effectively cast it once out of 4 or 5 attempts.
4. I like that it can make Monsters much more challenging. Hunting a Mythic Bugbear can be a lot different than a regular Bugbear.
5. I like the Mythic Spells too, in that I dont really need more spell compendiums, but this enhances the spells we already have.