I absolutely love it. I only discovered it about a year and a half ago, but in that time I've read a massive number of stories, in a lot of different fandoms. Mainly Twilight, but also Harry Potter, My So-Called Life, Firefly, Instant Star, Battlestar Galactica, Prison Break, and others that I might or might not be embarrassed to mention. (Kyle XY and Seventh Heaven spring to mind...)There are some real gems out there, if you're willing to look. And to make it even easier, there are some great communities that recommend stories, so you don't have to wade through the dross. Ficdiving on fanfiction.net can also be fun, and you never know when you'll find something amazing.
Some of the best stories I've read are fanfiction. Good writing is good writing. Some people never wrote a word before finding fanfiction, and then were inspired to create a masterpiece.
Also, I have been introduced to a varied community of interesting and talented people from around the world through Twific. Edwardville FTW!