A more well-thought review is to come, but I need to write down some of my impressions so I don't forget (?). 
 1) I loved this book and it's brilliantly written. I like how everything fits so well in P&P, without actually repeating or paraphrasing anything (the title of the chapters excluded). 
 2) I loved Sarah and the story of Mrs Hill, and Polly-Mary, and James, and Mr Hill, and Tol. 
 3) I loved how the narrator was kindest to the mistreated in the original book: Mary, MrCollins... She made them human. 
 4) Similarly, she made _human_ the other characters too, but in a different way: by giving them a new dimension that we don't see in P&P... and it sometimes came across as a bit mean. I liked to feel the carelessness and obliviousness people upstairs show for those downstairs, because I'm sure that's how it was (best example is how little Sarah and the rest of the servants are mentioned in the book). But (and maybe it's just a fangirl talking) I thought she made Elizabeth a bit more careless than she actually is... 
 5) The tone of the book is completely different to the tone of P&P because the story is totally different. That I like and understand: you need a bit of irony and lightness to describe the life of the rich(er), but you need to sober up if you want to be truthful the life of those downstairs. THAT SAID, I wasn't expecting to get THAT sober and that depressing. 
 (view spoiler)[6) I don't like when books get unexpectedly depressing. I like sad, I love sad, but I need to get ready for it. And the Third part of this book was a punch in the stomach. Very very bleak and depressing. And even though (again, spoiler) it ended well, I just wanted to cry. WHY? (hide spoiler)]