Oh, what fun!
It was a joy to see how Hugh and Serena first met and fell in love. Knowing already from The Duchess War that they'd end up HEA did help me when reading some of the things Hugh did to try to thwart Serena's attempts to disgrace Duke Clermont. If I had read this first, I might not have been so quickly forgiving. But then Hugh's such a giant goober and a softie that I might have forgiven him just as quickly anyway, lol.
Their interactions had a very Beatrice/Benedict vibe to them, and for once I didn't mind the insta-love at all. They were both evenly matched (well, Hugh might have been a little outmatched) and they both needed something from the other that they didn't even know they needed.
There's also a short little scene of Robert and Marshall meeting at Eton at the very end. It doesn't really add anything that we don't already know, but it was nice to see "live" as it were.