This is really not one of my favourite Heyers - in fact, until I re-read it, I had confused it from memory with Black Sheep - and I'm afraid that this recent re-reading hasn't done much to change my mind!
There are some good bits: I love the duo of Mrs Floore and Ned Goring (the latter very similar to all those quiet-but-competent secretaries who appear in other novels). There is some real insight into characters and motivation in places, perhaps an indication of the more mature novels (Venetia, A Civil Contract) that are to come. And I actually (to my surprise) found the relationship between Fanny and Hector quite charming.
But... I just find all the sparring and ranting and flashing eyes and heaving bosoms a bit tiresome. It's like a cross between Much Ado and Mills & Boon, and (for me) lapses into romantic novel cliche too easily despite GH's often-excellent writing. It ended up feeling very claustrophobic, perhaps because it's one of the most domestic of Heyer's novels: there is a very restricted cast of characters, little variety in settings, and no portrayal at all of wider society. (I realise that this is partly because Serena and Fanny are in mourning and wouldn't be jauntering off to balls and rout parties all the time, but I do think the novel suffers from this close domestic focus.)
I have problems with some of the characters, too. As I started reading, Serena reminded me strongly of Emma Woodhouse in her smugness, overt snobbery and unshakeable conviction that she was always right. Heyer does put in a few references to how well she gets on with the lower orders on the estate (always an attempt to show that someone is a good egg really 
) but they don't feel very convincing this time. When Rotherham's engagement is first announced, Serena's reaction reminded me of Emma's jealousy of Harriet Smith and how it "darted through her with the speed of an arrow that Mr Knightley must marry noone but herself!" What Serena wants, she generally gets. She then sort-of redeems herself by being all noble and self-sacrificing and agreeing to marry Hector just so Rotherham can marry Emily, but even here she comes across as quite selfish in her total disregard for what Emily really wants. I find the emphasis on Serena's bodily strength and vitality a bit wearing, too: for me, she combines the worst bits of Judith (contrary behaviour to spite her guardian) and Sophy (bumptiousness and masterful behaviour) without their redeeming charms.
If you can't already tell, I don't find her one of the more sympathetic heroines, but I'd be really interested to hear another point of view here!
Rotherham doesn't really do it for me, I'm afraid but I'm more of a Hugo Darracott or Alverstoke girl myself. His masterfulness made me bridle (although he does seem to suggest that he and Serena would enjoy a sort of companionship of equals at the end) and I got very cross at his proposing to Emily simply to spite Serena (he says something to Fanny about how at least she and Hector would never make mistakes from pique, I think). Throughout, he and Serena behave as if they are the only real people and everyone else doesn't quite exist fully or have feelings, and I found that infuriating.
And Emily is just feeble and I ended up feeling very sorry for how she is manipulated by all of the other characters without having any say of her own: her mother and Serena (and even Mrs Floore) try to manipulate her into marrying Rotherham, Rotherham tries (successfully) to manipulate her into jilting him, and even Gerard manipulates her into eloping with him against her will. Yes, she's a bit of a wet-goose, but I think she deserves better.
So... anyone actually like this novel, and can show me why I should reconsider my negative opinion?