Gilbert and Sullivan's The Grand Duke was their last collaboration, their least successful, and probably their worst; Gilbert's book is depressing, confused, mean-spirited and dependent on weird in-jokes that only he seemed to get (a lot of the plot hinges on axe-grinding about actors who are obsessed with their standing in the company). Gilbert was capable of better; the libretto he had written two years earlier, His Excellency, contains some of his best lyrics, and if Sullivan had agreed to do it, it would probably have been a huge hit. But Sullivan didn't come on board for His Excellency; he did come back for the inferior Grand Duke libretto, in another one of those shotgun weddings that he and Gilbert periodically agreed to (they would split up, write with other people, find they couldn't achieve the same kind of success separately, and semi-reluctantly get back together).
Sullivan's score isn't bad, just a little tired-sounding; he didn't like the book, didn't like working with Gilbert, and wasn't in great health. (Another problem was that of their regular actor/singers, the only one who signed up for this one was Rutland Barrington, so his part is inflated out of all proportion while several other characters have almost nothing to do.) Oddly enough, however, this is one of the few shows for which Sullivan took the time to compose the overture himself; like his overtures for Iolanthe and Yeomen, the Grand Duke overture is not a medley but a symphonic development of contrasting themes from the opera.
One of the more enjoyable songs from The Grand Duke was one that Gilbert actually cut after the first few performances, trying to shorten the incomprehensible second act (where the hero gets married to two women and is about to marry another before he's interrupted). It's sung by the Prince of Monte Carlo, who went from bankruptcy to sudden wealth when he invented the game of Roulette, the most sure-fire way to make people give him their money. (This was another of Gilbert's bitter in-jokes, this time about Sullivan's own tendency to lose money at the tables.) His song, with a mix of English and French lyrics and an unusually-structured refrain, is one of the few pieces in the whole evening that sounds like something G&S hadn't done before.
It was included on the D'Oyly Carte's recording from the '70s, sung by John Ayldon, who made a big hit of the number in a concert performance (the company never actually gave The Grand Duke a staged revival) and continued to perform it in recitals. I think it's usually included in the rare amateur and semi-pro productions of The Grand Duke.