My interest was held much more unevenly than the four stars would otherwise indicate, but the good parts were just that good.
Mitford makes no attempts to disguise her bias and interest, which makes for an eminently more readable biography than many. She's also far less meticulous with in-line sourcing than I've come to expect, which has the benefit of making things seem much more vivid and immediate while making it a bit harder to tease out where things came from and how much is Mitford's own opinion versus the recorded opinion of one of Madame's contemporaries.
Likewise, Mitford assumes a level of historical familiarity, both with the overall shape of events and specific movements/individuals, that could easily lose or alienate a reader. I consider myself at least decently read in French history, and I spent some quality wikipedia time with this book. A footnote or twelve would've been appreciated, though Mitford's fluency with her topic is also what makes it so eminently readable.
Can one describe a biography as a romp? Because I think this was, a bit.
Full disclosure: I read this in a mix of haze of thoroughly enjoying Mitford's novels and close on the heels of adoring the Doctor Who episode of the Girl in the Fireplace. I totally tried to pinpoint the spots in the narrative where the Doctor would've shown up.