Spamalot, the musical, opens on Broadway. “It seems safe to say that such a good time is being had by so many people (including the cast) at the Shubert Theater that this fitful, eager celebration of inanity will find a large and lucrative audience among those who value the virtues of shrewd idiocy, artful tackiness and wide-eyed impiety. That includes most school-age children as well as grown-ups who feel they are never more themselves than when they are in touch with the nerdy, nose-thumbing 12-year-olds who reside within.”