Gilliam movies always get negative reviews, because people suck. But they're always worth watching. I'm really excited for this one.
Actually, a glance at Rotten Tomatoes shows that you're wrong. Most Gilliam movies have gotten overwhelmingly positive reviews.
Here are just the ones I've seen:
98% Brazil
97% Monty Python and the Holy Grail
96% Monty Pyton's Life of Brian
94% The Adventures of Baron Munchausen
92% Time Bandits
90% Monty Python's The Meaning of Life
88% Twelve Monkeys
50% Fear and Loathing in Las Vegas
38% The Brothers Grimm
29% Tideland
With the exception of Fear and Loathing in Las Vegas (which I think deserves better), those ratings are spot-on. The Brothers Grimm freaked me the fuck out and I didn't enjoy it at all. Tideland is physically painful to watch; it should actually be lower.
The Zero Theorem has 54%. There's a good chance it's just a mediocre film.