Having played Mythic, I have only one piece of advice for GM's.
If your players are system optimizers and crunch junkies, Do not under any circumstances, allow them mythic. You probably need it for your own uses, if you plan on giving them any challenges at high level.
You've been warned.
As one of my groups is closing in on the end of wrath of the righteous...yea this is pretty true. If your group likes to exploit powerful combos, its going to get rediculous. Our sorceror basically has 'spend a few mythic points to end encounter' most of the time. Its sort of insane.
As an aside, I have seen a lot of success of dms granting specific mythic abilities to characters as story rewards, choosing them carefully, and making them relavent to the character. It still represents cool and often powerful abilities. But without the capacity to specifically choose everything, its far more contained even with optimized characters.