After 46 years away from New York, audiences at City Center finally have a chance to see what Jerry Herman and Michael Stewart’s 1974 flop “Mack and Mabel” is all about thanks to a new Encores! production. The result is bittersweet—both thrilling and sad. Jerry Herman’s score shines, alongside performances by Douglas Sills, Alexandra Socha, Lilli Cooper, and a top-notch ensemble, and the atmosphere of old Hollywood is superbly spun. Still, this is not a musical masterpiece, but a beloved curiosity—a good musical that almost works. Perhaps someday, it might.