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Personally, I really enjoy reading fanfic. If it wasn't for fanfic, I wouldn't have bothered with Harry Potter. I ended up reading the series just so I could figure out what the back story was to some of the fics I was reading
That being said, there's terrible fanfic out there just as there's terrible regular fiction. But at least the people writing terrible crackfics aren't trying to make money from it (50 Shades notwithstanding.)
There's lots of mega-famous fandoms like Harry Potter and Buffy the Vampire Slayer where the subject material can stand up on its own, but some of the most popular fandoms are ones where the source material was never fully developed. Take Stargate Atlantis. I love the fanfic, and it's built a whole fanon around itself that took what very little back story the show made available, and created whole histories for the characters, who actually are more fleshed out in the fic than they ever were in the show.
There's basically two realms of fanfic. One where the original story is really good and someone loved it so completely that they just had to share a story. And then there's the ones where the source material was weak, but the idea was good or a character was really good, and the fans just felt like they had to write a story of their own.