Oh I know, I actually enjoy Stephanie Barron's series of mystery novels where Jane Austen herself solves crimes (which is real-person fanfic), and Laurie R. King's Mary Russell Mysteries (which is nothing but Mary Sue fanfic about Sherlock Holmes).
I guess I hope that these stories I haven't read yet are of that higher caliber, because I have read mostly really badly written fanfic online. In fact I ended up giving up reading any of the online fanfic because:
1. obvious/predictable plot lines
2. people satisfying personal fantasy through a Mary Sue
3. incredibly long aimless WIP which never get finished, but at some point the author thinks they deserve donations...(along with their irritable demands for comments)

Maybe I was at the wrong sites, but I never read anything I enjoyed in the Buffy fandom, and I'm totally uninterested in Harry Potter fanfic, however I'll admit that really early on there were a few Firefly fanfic I enjoyed but it seemed to have become duller and more predictable when the movie came out (maybe that was just my perception? Or there was so much bad I couldn't find the good?). Anyway I don't mean to be snobby, I'm sure there must be good writers out there doing it, I just gave up trying to find them.