I'm wrapping up an E6 Mythic Legacy of Fire game (one session left). Players are topping out at level 6 + 20 bonus feats (APL 10-ish) with 5 mythic tiers (final APL 15-ish). The PCs became mythic in Book 1 and have gained roughly one tier per 2 effective levels. I deliberately excluded tier 6+ because of the crazy abilities it allows. In hind-sight, less is more when it comes to mythic. I could have gotten a similar game experience with only 3 tiers rather than 5.
For encounters, setting CR = APL + Tier is a good start. At higher levels, you may need to bump CR even more. Mythic is like an exponential multiplier. I can see how the higher the level, the more of a multiplier mythic becomes.
I made very few changes, but only because many of the tiers came before the PCs were getting E6 bonus feats. The PCs were very limited on Mythic feat choices since they needed to have the base feat as a prerequisite. As a result, I didn't have to face some of the shenanigans others have reported with things like Mythic Improved Critical, etc. However, I changed Mythic Power Attack to eliminate the additional multiplying on a crit, and it was a great change.
We are all having tons of fun, but the game has gotten a bit silly. I think the longest encounter in the entire campaign has gone 5 rounds. Most last 2 or less. In many, some PCs/creatures never get a turn. It isn't that the encounters are easy, they just don't last long and are very much on a razor's edge. And if the PCs "hold back" to conserve their mythic power for later encounters, the fights get really tough. There have been no deaths, but plenty of unconscious PCs, which made for some tense moments. It either goes really, really well for the PCs (ROFL-stomp) or really, really bad (near-TPK). There is nothing in between unless the PCs hold their mythic power.
When it comes to monsters, non-mythic monsters of an appropriate CR work fine as do those with a Mythic template added. True Mythic monsters will be noticed by the players, use them sparingly for good effect. Until your group sees it and gets used to it, be careful using monsters with Dual Initiative - they often get to go twice in a row unless a PC delays to prevent it. That being said, Dual Initiative is a great way to make a monster special and challenging.
Just to give you an idea of what you'll see out of a PC, here is the party fighter in action (fighter 6 + 20 feats/champion 5):
Gets to move speed as part of full-attack (speed 60 in full plate).
Option to spend 1 mythic power to increase speed to 110.
Make first attack at full BAB.
Make iterative attack with +5 bonus (aka at full BAB).
Spend 1 mythic power to make Sudden Attack at full BAB, add Tier (+5), roll twice, take highest, bypass all DR.
Spend 1 mythic power to buy a Standard Action.
Make attack at full BAB or Cleave if multiple enemies in range.
Against mooks or in conservation mode, the sequence is move, Cleave, 1 Mythic to Cleave again. And with Mythic Cleave, enemies can be anywhere within reach.