First of all, I found the Eloise and Colin chapters more disruptive than usual. I could just have particularly little patience for the various nonsense around weddings. However, I did enjoy the revelation of just what the modern day secret agent connections that have been hinted at throughout the series really are, so that wasn't a total loss.
The Pink Carnation and the Moonflower's mission was fairly straight forward, with the emphasis being on their relationship. I do rather miss the fact that the novella "The Pink Carnation in Love" was never completed, but Willig incorporates what's happened to Jane since we saw her last into the story in a way that isn't info-dumpy and is still easy to follow. And for a book that is primarily driven by the romance rather than by dealing with external obstacles, Willig also manages avoiding falling into the trap of creating more drama within the relationship than necessary. When Jack finds out that Jane has been withholding the information that she knows his father and sister, he is momentarily thrown, but it doesn't become a huge thing.
And spoilers in the next paragraph:
I was also pleasantly surprised that Jane and Jack carry on their espionage career after they marry. With Willig having said several times over the years that Jane's story would have to be the last one in the series, I thought she meant that Jane would have to settle down and be a proper early 19th century wife once she got a love story of her own, and I've often been annoyed at how many of the heroines have vanished from the field after their book was over (although a few of put in appearances closer to the action to conclude the series). Knowing that the Pink Carnation and the Moonflower's exploits continued together from Russia to Brazil makes me happy.
This was a far better conclusion to the series than I thought that I could expect, great fun by any standard: