I've written quite a bit of it. Not dozens of fics, like some people have, but... I just checked and I've written 19 Buffyverse stories, ranging from 1 to 7 chapters. And 28 more stories for other fandoms for a total of 47. Wow, I didn't realize I'd written that many.
I like to write it more than read it, but there are a few good ones out there. You just have to identify the good writers and focus on their stuff... and the stuff they recommend, which will also tend to be good. There's a lot of really bad fanfic out there, but in my opinion writing is never a bad thing. I never insult another person's writing or offer criticism unless it's asked for.
I just finished another story yesterday (not a Buffy one) for a fanfic writing contest, so I'm still at it.