Cotillion is a marvelous book: a delightful Regency-era comedy. As a romance novel, it's mostly a failure for me, by which I mean in no way to diminish its charm as a story. For me, a good romance develops the emotional connection and relationship between the protagonists. It should dwell in detail on the characters' feelings for one another, their hopes and desires. Cotillion doesn't do enough of this sort of thing to suit my tastes in romance.
This aside, I certainly recommend the work. The characters are well-drawn and well-rounded. They are likeable, if not always loveable, and the various non-romantic relationships -- friendships, relatives, and so forth -- are wonderful to behold. It reminds me of Austen in that the comic elements are stronger than the romance, which I've usually found the case in Austen as well. It is also like Austen in that it subverts certain romance tropes even while it adheres to others. It further helps in my case that it's subverting some tropes that I can't abide.
Unlike Austen, Cotillion is full of nobility and the London scene, subjects which Austen rarely touched upon. There's a fair chunk of Regency jargon in Cotillion that I didn't really know, which has the curious effect of making a historical novel written in 1954 feel more dated than Austen's early 19th century work does. I feel like I ought to read more romance novels that were written during the era so I can get a feel for whether or not the jargon is something common to other writers of the period and just not used by Austen's country gentry. In any case, I give this book a 9.