I'm glad to see these. Sondheim's reputation among cryptic-crossword aficionados is legion.
If you like this sort of thing, then you should subscribe to Harper's, and get a crossword much like this every month, created by Richard Maltby, Jr. (There's also the online puzzle in The Atlantic, but it's from Emily Cox and Henry Rathvon, so it lacks that Broadway connection.)
I'll put in on record I offically suck at crossword puzzles, not just the cryptic ones but also the lame entertainment-centric ones you get in the TV Guides and Soapie Magazines.
Still this is facinating (yet I of course will still not get 'em).