In sheer ambition and size, there's never been a bigger musical on Broadway than "Sweeney Todd," the latest in the astonishing series of collaborations between composer-lyricist Stephen Sondheim and director Harold Prince. As for achievement, well, it has to be said again—these men and their collaborators must be judged by the standards they themselves have set, the highest standards in the American professional theater. Judged thereby, "Sweeney Todd" is brilliant, even sensationally so, but its effect is very much a barrage of brilliancies, like flares fired aloft that dazzle and fade into something cold and dark. This "musical thriller" about a homicidal barber, a tonsorial Jack the Ripper in Dickensian London, slashes at the jugular instead of touching the heart….

Sondheim has been inching closer and closer to pure opera, and "Sweeney Todd" is the closest he's come yet. The show's texture is almost entirely musical, flowing...