Now 1/2 through, I'm finding it in many ways more compelling than Three Musketeers. Much darker, more nuanced politically, less of a fencing romp, more of a mature story of intrigue and drama. By all accounts this book was as popular at the time as the first. The next one in the trilogy, not so much.
To be honest, I think it has to do with the fact that I always pretty much loathed the musketeers, Athos and d'Artagnan especially. Heroic musketeers, my behind. Bullies and toadies, more like it. I suppose I had convinced myself they got their comeuppance somehow, and this book just didn't deliver. I think I failed to sympathise with pretty much anyone in that book.
Mind you, I am toying with the idea of writing Mylady's story one day. I always felt she was horribly maligned and I think it'd be fun to write a story of one of the great villains of literature and turn things upside down (or rather, calling them by their proper names).