Myself, I've read a grand total of four, which is probably four more than is healthy. Two were actually pretty decent:
The Terrible Secret of Animal Crossing, a hilarious (and at the same time terrifying) deconstruction of the insano logic of said game, and
The Ponies Discover /co/, in which the cast of MLP: FiM are exposed to 4chan's /co/ board and all the mental scarring that entails.
Then, however, there's the bad, and it is BAAAAD. I imagine some of you have heard of
My Immortal, an author insert Harry Potter fiction which is perhaps one of the worst things ever written. Honestly though, people don't read My Immortal for quality, they read it because they want to laugh their asses off at how completely and totally horrible it is, and to that end it succeeds spectacularly.
This happens. Seriously.
And then there's Cupcakes, another MLP fic that, while not terribly written, is quite infamous for it's rather... unseemly content.
^Let's just say, bad things happen to her.^ I'm not going to link it, you'll just have to look it up yourself.