I saw it in Philadelphia, when it was called Holly Golightly. The show has the reputation of being the worst flop of all time simply because it was so prominent, so anticipated, and then was shut down when still in previews. But it actually wasn't that bad. The problem was that its spirit and style didn't suit the Capote original at all: it had been turned into a conventional musical comedy with a dark side, when it needed to be as unusual and flavorsome as a Sondheim-Prince kind of show.
In other words, the same creative team might have come up with a perfectly acceptable piece using completely different material. Instead, they chose this all but untranslatable story without thinking how best to musicalize it on its own terms. Further, neither of the two leads had anything substantial to play, and the support was all over the place. I remember a party scene in which the chorus guys glared at each other and sang about "dirty old men" as if Bob Merrill were a child who had just discovered a naughty term and was using it to shock the grownups. In the same scene, Holly and Mag Wildwood sang a duet vcalled "The Home For Wayward Girls" that had nothing to do with the action.
The whole evening was like that--snippets of things that never cohered. But it wasn't boring. It was the sort of show that you could enjoy even while you knew it was terrible.
Ethan Mordden's book on sixties musicals, whose title I forget, has several pages on it. Ken Mandelbaum talks about, too, in Not Since Carrie.
By the way, Breakfast at Tiffany's played only three previews in New York. The Wednesday matinee was cancelled.