I've never read P. D. James, although I know her excellent reputation as a mystery writer. I also avoided the tidal wave of books based in Austen's world, not out of a sense of snobbery about spin off fiction (I've read tons of Star Wars and Star Trek spin offs), but because of my deep affection for Austen. I couldn't see anyone writing in her world with the same sense or sensibility. After the wave crested and many people had read many of the books, I chose two that most folks seemed to think stood out from the crowd...and they sat on my TBR shelf for months. Then Masterpiece Theater showed the BBC adaptation of Death Comes to Pemberley and I had to read the book before I watched the show. 
 I had a mixed reaction. I truly enjoyed James'imitative voice and her clever inclusion of characters from Austen's other books in this world through gossip and back story. I particularly liked her sly pokes at the original material by pointing out some of the inconsistencies in emotional through lines (why did Charlotte marry the odious Mr. Collins?) and filling in the gaps with explanations. However, I found the central story or supposed mystery boring. I guessed a couple of the main plot points early on and read primarily to confirm my predictions. 
 From a craft point of view, I found the characters rather shallow. No amount of telling me how horrific the murder was convinced me that any of the major characters had more than a surface reaction. The endless repetition of the "facts" by various people for an initial police inquiry, then a coroner's inquest, and finally a trial (not to mention the various conversations between characters), was just plain annoying. So I read the first two-thirds with appreciation for the style, but bored by the story, and raced through the final third as various characters told us the missing pieces--which turned out to be deliciously twisty. Too bad some of the that fun couldn't have been included in the earlier bits. The last third rescued the book from an "OK" rating.