Did you know, it was originally to be a 12 part miniseries for BBC1? Considering it was cut nearly 3 times over, I'm very happy we have a 4 hour miniseries that slimmed down with relative ease. And while large sections of the story feel rushed, it is only keenly felt once Angelica has married her "hero of the moment" and you wait for the inevitable.
The Devil's Whore follows the life of Angelica Fanshawe, a fictional character set during the English Civil War. While Angelica is historically inaccurate, the men she loves, marries, and buries are all historical (minus Henry Fanshawe, but that's to be expected), and highlights the political and moral duality of the Civil War period.
And then, of course, there is the simple fact that its 4 hours of beautifully acted costume drama. Fassbender, Simm, and Dominic West put in brilliant performances; while Andrea Riseborough holds her own as the haunted Angelica. I'll admit it, I'm not so prideful as to not be able to: I first watched The Devil's Whore simply for Michael Fassbender, and it was a plus it was about the English Civil War; so few period dramas cover that span of history. And yet, as I watched--and certainly as I rewatched--I found myself lost in all the characters: the idealistic, the tradition-bound, and the foolishly hopeful. I realized that John Simm's profound, yet quiet performance as Edward Sexby is inspired, and that he can certainly handle being a dark and broody leading man. To this, Fassbender became the plus to a beautifully acted and written period drama. 
Just think of what 12 hours would have brought us. I throughly recommend a watch of The Devil's Whore for anyone in love with period dramas. And it is certainly a watch for those who want something a little more gritty than an Austen adaptation, or even a Bronte one.