I have been a huge fan of Jane Austen for years, and I've read and reread all of her books. I must have read Pride and Prejudice 20 times at least. I've also watched nearly all of the adaptations and read many books based on P&P - modern and historical, mysteries, comedies, even crossover fan fiction. I even recall a cartoon called Wishbone doing a short bit.
All of this is, of course, fan fiction, though the term isn't usually used. "Clueless" is fan fiction in the form of a movie (although of Emma, rather than P&P). Bridget Jones Diary is a fan fiction novel.
What makes a good adaptation of Pride and Prejudice? For me, it has to capture the spirit of the book. The romance is lovely, of course, but so is the character development, the humor, and the close relationships with family and friends beyond the romance. Darcy and Lizzie aren't broken Heathcliff sorts. They have loved family members and friends, they are intelligent, decent people who generally enjoy their lives. They each are a bit lonely and a bit too arrogant and sure that they perceive everyone's character and true motivations. They don't need each other in order to change who they are, but to be each others' match, to broaden each others' horizons, and to be happy.
So how is Pride and Modern Prejudice? Not bad. I liked the main characters. I laughed a lot. I had a good time. I didn't have the worry for the characters and the deep satisfaction of seeing them finally together that the original always gives me - but that is a high bar!
It was fun to see how the author approached various plot points in her retelling - she pretty much always went for the funny and offbeat option rather than trying to stick too close to the original, which in many places wouldn't make sense in modern times. The plot points didn't always make a lot of sense, and many of the characters shared little but a name with the originals. I was also sorry the sex was closed door - I don't need a lot of sex in my romances, but sex is an important part of how characters falling in love relate to each each other and I miss it when it isn't there at all.
So I would recommend this with the caveat that it does better with fun and humor than with plot and character.