So today I ran my first mythic game for my "throw away" party. The party is all 15th level with 2 mythic tiers (next week we'll add more), and consists of a bard, summoner, oracle, monk, inquisitioner, fighter, and cleric (the party was built using a 18, 16, 16, 14, 12, 12 stats and 300K worth of gear). We did some sample fights last week to get a feel for what they could handle (golems no, dragons yes).
This week we did two fights, one vs. 5 mythic Ettins, and the second vs. 2 mythic fire giants.
DM's feedback:
From my point of view, epic ettins seemed a little week in the saves and hit rolls, but they were able to get a few hits on the oracle and the summoner, however, the party messed up the ettins in 3 rounds without taking a ton of damage. The inquisitioner using his mythic move attack move ignore DR allowed him to take down two damaged ettins in the same round (he was focused around criting and fought with a keen rapier). Mythic initiative gave crazy high numbers, but it didn't matter in the end. Ettins poor saves pretty much made them toast for will saves (which made the bard happy) and the summoner was able to baleful polymorph his ettin while using the archmage mythic path ability allowing him to just not do a defensive casting check. We also had one of those great moments of "I rolled a 1 and I still hit him?" which made the fighter very happy.
Second fight vs. the fire giants was not a contest at all, as it seemed the DM curse effected me fairly heavily and I wasn't able to hit anything before the party tore them apart.
Next week I hope to have my final play test on this with my party vs. a CR21 non epic critter, 5 or 6 critters adding up to a CR21, a group of CR17 mythic monsters, and a CR 25 mythic creature (just for fun).
From my experience, the CR guidelines break down a bit when you get past 4 - 5 players in Mythic play. I'm upping to APL+Mythic Tier+2 for a "normally challenging" encounter. I'll be interested to hear how the rest of yours goes.