Man, if you want to see a fandom wig out, just insert a bit of reality into their happily ever after narrative. This is not a one star book. It's a more than competent piece of fiction. Fiction being the key word. P&P isn't real, folks.
Jane Austen books are fun because they aren't demanding. Good and bad are pretty clearly outlined and the endings are emotionally satisfying. McCullough adds nuances to the characters and complicates their lives because life is complicated. Much more to my liking. Also, to be honest, I've never liked Mr. Darcy all that much (not to be confused with Colin Firth as Darcy - a totally different creature), and found him controlling and cold. He never hints that he's going to accept Lizzie's background. He just goes about his approach at catching her in a different way the second time around. What happens to them in this book is pretty much the outcome I'd expect for their marriage.
However, as much as I liked how the old characters were made more human, I thought Mary was incredibly unrealistic! If McCullough weren't such an established writer, I'd be screaming "Mary Sue!" I mean the only thing going against Mary is that she's naive. That's it. No other flaws. It's a bit like making an otherwise perfect woman klutzy just to remind people she's human. Also, Charlie. Angelic teenage boys are hard to sell to me.
Overall, I didn't find it all that compelling of a story, but it's a fun enough romp. Maybe Regency fiction just isn't something I dig anymore. Still if you are looking for a P&P sequel that is different and aren't emotionally invested in the fairytale ending of P&P, then you will probably enjoy it.