After having played a few mythic games, both in the play test and with the current rule set, I've made an interesting observation: Mythic rules are positively excellent for small groups!
A friend of mine played a level 8 tier 4 mythic duo (a barbarian/champion and a sorcerer/archmage) in an otherwise standard adventure (that ended with a mythic boss fight). In almost every single non-mythic encounter, we performed as well as a four-man group would have thanks to our mythic abilities giving us a little extra durability and, most important of all, an increased action economy. As long as we had mythic points, we could take on just about anything*.
It wasn't until we encountered the final boss, a tier 4 guardian in her own right, that we were completely trounced due to our inability to really hurt her. Though this sucked for us, it only further strengthens my observation that mythic is great for small groups, or even solo characters.
So I just thought I'd share that with the community at large in the hopes that your small group games might be made better for it. Also, feel free to discuss the merits of my observation, and voice whether you agree or disagree with my findings; or simply share your own mythic, small-group experiences.
* About as well as a normal four-man group would have.
EDIT: And in fact, if you are familiar with the CR system and how it is balanced (or have read The GM's Guide to Creating Challenging Encounters) than you might realize that in the above example, the two level 8 tier 4 characters are equivalent to a CR 10 encounter. You know what else is equivalent to a CR 10 encounter? Four level 8 tier 0 characters.
See how that works out, not only in practice, but mathematically as well? This leads me to believe that applying mythic tiers to your games would be a great alternative to having the players play multiple characters (which might slow things down), to including annoying GMPCs (which players tend to hate), or to having to adjust encounters (which GMs tend to hate).
I'm very interested in how this is working out for everyone. What are the pitfalls a DM needs to be aware of when letting two mythic PCs tackle an unmodified non-Mythic AP, like the updated Rise of the Runelords one? I'm especially concerned about the abilities that have a clause vs. non-mythic enemies (like Mythic Hexes). How do you deal with them?
Also, how would you handle the first levels? Start the PCs off at level 1 and hand them 2-4 mythic tiers immediately? Or start them out as lvl 2 tier 1 and try to keep the 2 levels : 1 tier balance?