I'm fascinated by the lukewarm response this show's received. It feels so refreshing to have a show a) tell you what it's going to do and then b) do it. It spends the first three episodes establishing the characters and the conceit, and then follows through on everything a narrative is supposed to follow through on.
Every major character has a "core trauma" that they must confront and move past. They ingest pills which put them into dreamlike states where they can fuse their trauma with mythological encounters, which lets them face their problems head on in a more abstract sense. Just about to the letter how Joseph Campbell explained the purpose of myths and legends.
Even the non-dreaming characters face larger-than-life scenarios they must overcome in order to overcome their own core problems. Without spoiling it, Dr. Mantleray (Justin Theroux) has a hilarious and way over the top problem that he confronts in an equally ridiculous manner.
Which reminds me of another facet of this show: it's not afraid to let you (or make you) laugh. Unlike Bojack's Flip McVicker, who says before his premier, "This show contains no intentional humor."
It's a tonic for people tired of Prestige TV. The "it gets good" syndrome, where we have to wait a whole season for any kind of "payoff" to hours of wheel-spinning, setup, and dissolution. I'm tired of mysterious characters doing weird things and waiting eight episodes to find out why. Plot is not a puzzle. I want to know the characters, to care about them, to know in clear terms what they want and watch them struggle to achieve that. Which, I think, Maniac delivers on.
Also, like a lot of character dramas, the most direct "about" is your investment in what happens to/between the characters. But I think you could fairly say the show is an exploration of how we come to grips with limited control over our minds and emotions.
The Leftovers was basically Lost done right, very very clear from the onset that it's about people's reactions to the craziness than the underlying reasons for it. Maniac feels like it has some unnecessary stuff tacked on, the premise has a Charlie Kaufman copycat element to it all that feels like a necessary gimmick as opposed to something which naturally fits into the world.