I'm not sure I'd call myself a Terry Gilliam fan. I think I've seen all but two of the films he's directed, but I don't have strong feelings about most of them. I'm somewhere between being indifferent to his work and enjoying his films but mostly forgetting they exist. I don't list him as a beloved filmmaker or think about his movies often. I think the film of his that I've been the most attached to is 12 Monkeys and I'm not sure I've watched that in 20 years. That said, I at least feel well-versed enough with his traits and foibles to compare the experience when watching one of his new films. And I've been putting off The Zero Theorem for about 6 years, so I decided to finally rectify that.
Based on my experience, I feel like the closest analog to The Zero Theorem is Brazil. Another film about a dystopian future society awash in bureaucracy and consumerism, a lead character whose reality and dreams come into stark opposition, a growing obsession with a strange woman, events spiraling and leading to a confrontation with those in power, and dark endings for the protagonist, who ends up mentally locked-away. Though Gilliam didn't write the film (though he did rewrite the ending from happy to dark, in a nice reverse of the Brazil situation), it still carries many of the earmarks of his previous work.
It has a different feel than his '80s films. Brazil has a certain grain to the look that ties it to Time Bandits and Jabberwocky and The Meaning Of Life. This film is clean; pure Gilliam but smoothed out and modern. It may be one of the most mature things he's made. He may not have crafted it from the beginning like most of his films, but that may have actually improved his vision.
The cast he chose obviously did a lot of the heavy lifting. Christoph Waltz has shown an incredible aptitude for compelling characters and was the best possible choice for the role. His sad-sack character, devoid of pleasure or meaning in life, could have been an annoyance if played badly. He feels understandable and believable. I had only seen Melanie Thierry in Babylon A.D., where she was good, though not overly likable. She shows range here in a way I didn't expect. If Billy Bob Thornton and Jessica Biel has been cast, which almost happened years before, it would have been a disaster. The movie is pretty much grounded in the actors making it work.
It's hard to say too much without just explaining the movie or talking about the ending. There is a bit of interpretation to the finale, but it's not really worth discussing. But this is a film absolutely worthy of your examination. I'm sure, like other Gilliam films, it'll drift away from my mind soon enough, but there's something here worth holding onto.