I actually read Mr. Darcy's Diary on the ferry this past weekend and really enjoyed it. If you are going to read any of the new Austen spin-offs, I'd recommend this book over reading Austenland (see review below).
Mr. Darcy's Diary is Pride and Prejudice retold from Mr. Darcy's perspective. The novel fills in some of the gaps of what Mr. Darcy was doing when not on center stage in Pride and Prejudice and also gives his prespective on events. Most satisfying of all, it gives a glimpse at the end of married life for the Darcys - and it was wonderful!!
While nothing can compare to the original novel, this book was very enjoyable - I recommend it for Jane Austen fans out there. I read on Austenblog that Amanda Grange has also written Mr. Knightly's Diary and Captain Wentworth's Diary. They are already published in the U.K. - I can't wait until they come to the U.S.!