With "bare bones" (read: no magic items) PCs it's pretty manageable, though as usual, the more full casters you have in a party the less manageable it becomes. It's still more manageable than earlier editions. As [MENTION=12377]77IM[/MENTION] states, it's easier with players who aren't 20th level murder hobos (that needs to be a class already). The more players invest in a setting, a party, a game, the more they'll self-restrict and you won't have to throw squadrons of Solars at them to keep them in check.
Like most editions, the more magic items you add to the mix, the more out of whack things can get, depending on which magic items they are, who is using them, and how cleverly someone can use them.
As others have said, I've played at max level in 5E, but not for long 6-8 sessions tops, wrapping up the final bad guys, saving the world, heroically sacrificing themselves, settling down.
As a player and as a DM, max level in 5E is a little underwhelming and doesn't really feel much different than anything 15th or higher.
It's not 3.5 level 20. And all that that implies.