I think the novel is unusually candid and negative about marriage, oddly enough. Although we've gone through dozens of happy marriages in this novel, Heyer is taking care to show us that many, possibly most, marriages were disappointing if not outright miserable. There was no expectation of happiness, or of love. And while the men might carry on regardless, the women were trapped.
The very qualities which had fascinated Denville in the girl offended him in the wife, and he set himself to the task of eradicating them.
It's chilling, isn't it?
"She told me that I must be the last person to wish to see my son make an unhappy marriage, for that was what I did myself. I must own, Kit, that I was very much touched."
His pleasant grey eyes looked steadily down into hers, the suggestion of a smile in them. "Tell me, Mama, were you so unhappy?"
"Often!" she declared.
Kit may not have understood his mother's misery, but Evelyn does.
"She was seventeen when my father married her! [...] My father - years older than she was! - fancied himself to be in love with her! Love? He was dazzled by her face, and her captivating ways, and had no more love for her than I have for Cressida Stavely! Everything in Mama which makes her so lovable he disliked! Kester, he drove her off - pokered up when she showed her affection, in that impulsive way she has! It was not the thing for Lady Denville to allow the world to suspect she had a heart!"
She lets slip to Cressy how terrible it was, later on, and by accident:
"But in every other way it is most agreeable, I find [, to be a widow]. In fact, far more agreeable than being a wife! At least, it is for me, but not, of course, for you, dearest!" she hastened to add, with one of her lovely smiles. It faded; she looked stricken all at once, and older; and said: "I was forgetting. You see, it is of no consequence." Two large tears welled over her eyelids, and rolled unheeded down her cheeks.
The irony is, of course, that at the beginning Cressy is actually being urged into a similar society marriage: it was a very good match, and [...] Cressy, at the age of twenty, and with a dowry of only £25,000, would be a fool to draw back from it. There's absolutely no reason that she and Evelyn will not prove to be so unfavourably matched, and so miserable, as well. But that's all right, because it isn't for their own benefit, but for financial and dynastic reasons.
Kit was ready to swear that if [Evelyn] married Miss Stavely he would never use her unkindly, or wound her pride by blatantly pursuing some other female. Whether he would remain faithful to her was another and more doubtful matter; but he would conduct his affaires with discretion. Presumably Miss Stavely, no schoolroom miss, but a rational woman, entering openly into a marriage of convenience, was prepared for some divagations, and would demand no more of Evelyn than the appearance of fidelity.
Even Kit, who supposedly loves Evelyn more than it's possible to love another person, does not have high expectations of him, nor of the marriage. And Cressy has no better expectations:
"We were agreed, were we not, that only candour on both our parts could make our projected alliance tolerable to either of us? Your reason for wishing to be married is your very understandable desire to become independent of your uncle; mine is - is what I feel to be an urgent need to remove myself from this house - from any of my father's houses!"
Meanwhile, Lady Denville ends up using marriage to reestablish her social (and financial) position, despite the fact that she feels only a slight affection for Sir Bonamy, who throughout is shown as something of a buffoon: Owing to the height and rigidity of his collar-points, and the depth of his Oriental tie, Sir Bonamy could neither shake his head, nor nod it. When he wished to signify assent, he was obliged to incline the upper part of his body in a stately manner which frequently exercised an unnerving effect upon strangers already awed by his size and magnificence.
And this is despite the fact that she has to trick him into proposing, teases him that he doesn't want to, and only does so because she can't bear to live with other women, can't pay her bills, and feels that the title "Dowager" is terribly ageing!!
And she has effectively to ask Evelyn's permission to marry Sir Bonamy. That is, she doesn't really, but she can't afford not to secure his approval, and has a job to secure it. In the end Evelyn is only won over for practical, financial reasons. His own love match notwithstanding, he sees marriage as primarily a financial and legal transaction.
On the other hand, it's interesting that while our heroines are gradually getting older (thank goodness) this is our youngest hero for a good while. Obviously that makes me more convinced that it isn't really about him 
but 24 is very young. I think only Sherry and Vidal are that young, and is Harry Smith (who doesn't count) only a year older? In any case, Kit is only four years older than Cressy, so it's already more of an equal partnership than we're used to - Heyer falling out of love with the Avon/Leonie model again.
A minor gripe is that Heyer tells us too much in this novel. In better novels she shows - here we have three chapters of exposition before anything happens. It's not good.
So essentially I think it's good, but not for the usual Heyer reasons. She's out of her groove and we need her back. Frederica next, thank goodness!! Back to rich older man and irrepressible younger girl, supported by awkward teenagers and comic children - we know where we are there.