The best auditorium in Boulder is at the UA Village Theater. One of its four auditoriums has a giant curved screen (the second-largest in the state), with super picture and SDDS digital sound. The auditorium seats 700, but it's usually not too full, especially at a matinee. It shows a good mix of blockbuster action movies and indie/crossover films.
Caveat: The other three screens are very small.
