I picked this up at the gift shop at the Jane Austen House museum during my recent visit to England. It was a fun and well-written, if necessarily predictable, retelling of the Pride and Prejudice story from the point of view of Mr Darcy. As such, I don't have too much to say about it - if you haven't read Jane Austen's original, I suggest you go and do that rather than read this first. Then you can pick up this book and enjoy reading the story again, from a slightly different perspective, complete with the "happily ever after" chapters at the end, in which the author gets to indulge her fantasies (in an occasionally un-Austen-like manner) about Elizabeth and Darcy's marriage.
I liked Darcy's Diary far better than the hideous and embarrassing Mr Darcy Takes a Wife, which I do not recommend you pick up if you want something told in the same style and spirit as Jane Austen's stories. If only I'd taken note of the title of the other book recommended by the Jane Austen House employee - I'd enjoy reading more P&P sequels, but it's hard to figure out which ones are worth my time.