Even if you can't be there for an opera (opening with a gala to end all galas every sacrosect December 7), attend a concert or dance performance there or at least visit the little museum (left of the theater), if only for a chance to see a strand of Mozart's hair, Toscanini's baton, or a peak into the sumptuous Neoclassical auditorium with its six tiers of balconies and galleries.
It is worth visiting any form of performance there.
The dance performances at the theater are not worth seeing.