Danny Fingeroth, who knows a thing or three about super-heroes, writes the following about the musical, It's a Bird…It's a Plane…It's Superman!…
I saw the show in 1966 at a Wednesday matinee and remember being disappointed that it didn't actually seem to be about Superman. Jack Cassidy played a character who wasn't part of the Superman supporting cast from the comics or the TV series, yet he was very much the star and the center of attention. That could be why the show wasn't successful. Whatever you may have thought about, say, the Batman TV show, it was very much about Batman and/or his villains, not about a guy who wasn't any part of the character's core mythology or well-known supporting cast.
Yeah, that's another thing wrong with it. No Lex Luthor, no Jimmy Olsen…and I think Perry White only has about three lines. Jack Cassidy was then a big deal on Broadway and it may be that since he was available, they were more interested in building the show around him than the relative unknown who'd be playing Superman. It throws the show even more off balance when it's done without a major star in the role because then there's really no reason to devote so much attention to that character.
By the way, everyone: Bob Holiday, who played Superman in that show, has a website that contains almost no information about the original show. He doesn't even have a plug up for his book on the experience, Superman on Broadway, which may or may not even be available these days. But if you do go to Mr. Holiday's site, check out the video clips of him teaching Steve Allen how to fly on the game show, I've Got a Secret. And thanks for the message, Danny.