Over the past few years I've reviewed for PW several sequels to classic novels and, earlier this week tackled my first classic monster mash-up for the magazine. It has not gone well.
The vast majority of classic sequels have been for Jane Austen novels. Save one, they've been disastrous, and the sole non-disaster was, at best, no better than "not bad." I try to maintain an open mind for every book I review, but must admit that I am not predisposed in favor of these sequels because I'm a purist. I refused, after all, to watch Clueless for years after it was released to critical and popular acclaim, after all, because no matter how much Amy Heckerling loved Emma, she dared to update what for me already is a perfect story.
As for the myriad of sequels published over the last several years, I find it incredibly presumptuous for any author to believe she knows what Austen might have done with her characters after the books she wrote. Had Austen wished to further their stories, she would have. My guess is that she would not be pleased that Darcy hid his erections from his jovial, knowing father-in-law any more than she would want Lady Catherine DeBurgh to continue plaguing future generations of Bennetts and Collinses and Bingleys. 
Why these books sell really surprises me; that one author has parlayed Pride and Prejudice into eight sequels to date astonishes me, particularly the one I reviewed, which featured the children and grandchildren of a secondary character in the original. A less annoying read was one of the Sense and Sensibility sequels, but it still frustrated me. While it tackled a social issue I can imagine Austen taking on, Austen's sensibility was sorely lacking in the sequel's prose style, resulting in a mostly plodding effort.
Those are my thoughts on classic sequels. While you might think I harbor similar ideas about monster mash-ups, I don't. I love parody, which is the reason why at AAR I created our annual Purple Prose Parody Contest that in its later years encouraged mash-ups. 
When Pride and Prejudice and Zombies came out last year and received a high mark from Entertainment Weekly (which propelled it from a minor release into a major one that eventually spawned other monster mash-ups), I loved the idea of it so much that I pushed it like mad at the bookstore, although my own effort to read it ended with chapter three. Had it been any mash-up other than P&P, I would have soldiered on. 
On the other hand, what I've read so far of Sherri Erwin's Jane Slayre has whetted my appetite for more. I love Brontë's original, but it's not at the top of my all-time keeper list, so I'm less invested in it retaining its purity. Erwin's love for Jane Eyre as well as her skills as an author come through so strongly that it's easy for me to get into the fun of a Jane plucky enough to slay vampires and the idea of Rochester's crazy, attic-bound first wife as a monster.
That said, the review I sent in to one of my PW editors yesterday was for a book lacking in any of that sense of fun. The book was such an unmitigated disaster that it took me an entire week to get past the first hundred pages. I can't get into more detail until the book is released, at which point I'll provide a link to my review, but right now it's in serious contention for my worst book of the year.
While both of my PW editors know that I'm probably not the reviewer for future Jane Austen sequels, I remain open to the idea of classic mash-ups, although I wonder about their shelf life - for every Jane Slayre there's a Sense and Sensibility and Sea Monsters. Of course, I remain happy to read and review humorous paranormal historicals. Not only am I a fan of Gail Carriger's Parasol Protectorate series, both Jill Barnett's Bewitching and Rebecca Paisley's Basket of Wishes, which were released in the early/middle 1990s, sit on my all-time keeper shelf.
