From three levels of play (so far).
Group: 6 PCs: human berserker barbarian, human moon druid, half-elf valor bard, human champion fighter, wood elf thief rogue, mountain dwarf evoker wizard. All 3rd level.
Encounters so far: goblins, hobgoblins, giant rats, snakes, crocodiles, lizardfolk, zombies, gargoyles, bandits, thugs, cultists, and a young black dragon.
Char Gen: Point buy, average hp per level. (Important).
Magic Items: Two +1 weapons (longsword and dagger), a few potions and scrolls, and a couple utility items (rope of climbing, periapt of health)
General impressions: I can usually get the one of the three frontliners (bbn, ftr, drd) down to single hp if not downed (the fighter usually drops once per session, the other two can usually mitigate the damage thanks to rage and wild shape). I've dropped the bard and wizard once each as well. (The bard is our primary healer). The rogue just hides/snipes most turns. I find rarely is any one encounter "deadly" but I can usually tax them with smaller encounters and spreading the damage around.
Thanks two players missing, I've also run these PCs in four- and five-character parties. I find that encounters that they were taxed more (more short rests, more HD used) than the six person party, even adjusting encounters for party size. I think that's key. Even using the DMG guidelines, six person parties just bring a LOT more to bear. I'm reaching the point where I'm going above the challenge level (CR 4 for 5 for a boss-fight) and I almost never use solo monsters except for "easy" encounters. (I have yet to use a monster with legendary/lair actions though).
For me, the thing that is keeping things balanced is average hp. Our fighter has 26 hp, our barbarian 35. I can still crit-them into single digits or downed using relatively normal monsters (a beserker NPC one-shotted the fighter at 2nd level on a crit). Similarly, none of them have super-high to hits, so a creature with an 18 AC (like a hobgoblin) is still hard to handle. Nothing has been a TPK yet, but there have been enough dropped PCs that the group is careful with their resources.
I wager things will get crazier when they hit 5th level; two PCs will get extra attacks, and the wizard will learn fireball. They'll also have their first stat-bump/feat. We'll see how crazy things get then.