Both Darcy's Story and The Confession of Fitzwilliam Darcy are (as one might glean from their titles) retellings of Jane Austen's Pride and Prejudice from Mr. Darcy's point of view. Pride and Prejudice has been so well-beloved by readers for generations that it is perhaps hardly surprising that there are so many adaptations and reimaginations of the story; I have also read and enjoyed Pamela Aidan's trilogy, which is written with the same premise, and there are many other examples, including both retellings of the story from Darcy's point of view or continuations of the story after the ending that Austen gave it.
Given that any retelling of the story from Darcy's point of view is limited by Austen's original text, it seems unfair to ding Janet Aylmer's Darcy's Story by criticizing it for a lack of originality ... but it's simply impossible not to do so. Aylmer copies large parts of her text from Austen's novel, particularly the dialogue, and her Darcy is the same reserved, uncommunicative gentleman that we meet in Pride and Prejudice. In other words, I don't feel any closer to Darcy for having read this - apart from one or two details about Georgiana, Darcy's Story doesn't add anything to the characters that is not present in the original text.
Especially compared to Austen's wit and writing style, the passages that Aylmer has written suffer in comparison. The text drags on, perhaps especially because sections of it are extremely repetitive - and especially when Aylmer repeats quotes from Austen's text, it's unnecessary, as the large majority of her readers are Austenites who are familiar with the fact that Mr. Darcy is a reserved gentleman who envies others their ease at social interactions.
Mary Street's The Confession of Fitzwilliam Darcy is a much better read - she doesn't quite manage to capture Jane Austen's language, or write with the same wit that Austen uses, but her version of Darcy is a much more interesting character. He isn't a cardboard silhouette of the original Pride and Prejudice Darcy, as Aylmer's character was - there are scenes that didn't occur in the original novel and there are new insights into Darcy's character. Darcy here is much more passionate, and the break that comes in his character after Elizabeth's refusal of his first proposal is quite convincing, and almost as painful to read as it must have been to experience.
Instead of leading the reader through the story step by step, as Aylmer does, Street's novel works very well for the reader who has already read Pride and Prejudice and knows the story from Elizabeth's point of view - several of Darcy's misapprehensions are diverting on that basis. Likewise, Street does a better job of managing dialogue, and while there are some lines that are direct quotes from Pride and Prejudice, she manages to paraphrase or work around having to repeat entire speeches. In some cases, this style works better than in others - juxtaposing Austen's words with Street's doesn't make for a seamless, perfectly flowing narrative - but on the whole, I found it infinitely preferable to wholesale quotation of large portions of text.
As a confirmed Austenite, I don't expect that I'll ever find a rendition of that pleases me as much as the original. Pride and Prejudice. Darcy's Story is one that I would skip, if I had the choice of reading it again, but on the other hand, I did find The Confessions of Fitzwilliam Darcy to be an engaging read. It brought something new to the narrative, instead of casting the story from a slightly different camera angle, and I had pleasure in the insights into Darcy's character that I gained from reading this book.