My opinion from the start was that there would be a strong element of unpredictability in power levels due to so many rules and options being added, some using completely new mechanics. We were told that this was necessary to 'tell the story they wanted to tell'. Hogwash.
We dispensed with the Mythic rules, in part because I didn't want to have to deal with the over-complications that would inevitably ensue, in part because my players wanted to finally get to use their capstone ability, and in part because there seemed like genuine interest in how playable the AP would be without them.
Party of 4, 20 point buys and the characters leveled at the predetermined times as well as gaining levels at what would have been Mythic Tiers 1, 3, 5, 7, 9 and 10. Meaning characters would level faster and would top out at 26th level, though no character was allowed to have more than 20 levels in a particular class. To keep the power curve more even at higher levels, I added a single 'epic' rule:
At each level beyond 20th, you get to reroll a single d20 X/day with a bonus of +X where X equals your level above 20. In other words, once a day with a +1 bonus at 21st, twice a day with a +2 bonus at 22nd and so on.
This far its worked out superbly, the only problem having been the work necessary to disassociate mythic threats with some of their mythic aspects. The players feel suitably epic, are well challenged and the story is being told perfectly well. Moreover, the power curve if infinitely easier to predict which makes life much easier for me as the GM.
Wrath of the Righteous = tremendous story and epic campaign.
Mythic Rules = utterly unnecessary complication which may well have ruined the AP for many players as we're seeing in these forums.