I am not a fanfiction reader. For me, I'm content with the worlds the original author created, and don't have any interest in reading another person's spin on it. But there are lots of people who are passionate about fanfiction, and love that it allows them to stay in the story worlds that they adore, long after the final book has been published. And that it totally awesome!
What I have found that I like though, are books about the fanfiction community. I've only read two, but liked them both immensely:
I'm actually getting ready to read Carry On by Rainbow Rowell (sitting by my keyboard as I type this), which is based on a fanfiction story that Cath (the main character from Fangirl) wrote. I loved the snippets of Carry On that were shared in Fangirl, so I can't wait to dig into a novel inspired by fictional fanfiction. (okay, my brain is starting to hurt, lol!)
What are your opinions on fanfiction? Love it, dislike it, or write it?