In the geek hierarchy, writers of fan fiction fall just below amateur scifi/fantasy writers. Unless, of course, you happen to write erotic fanfic, or (Og forbid) write yourself into your fanfic, write yourself into erotic fanfic, or write yourself into erotic, furry, Star Trek fanfic.
I think fan fiction gets a bad rap. While much of it is painfully bad, so is much of any sort of writing. I consider fan fiction to be either an opportunity to write with training wheels (so to speak) or the fiction-writing equivalent of an English sonnet. And, I must admit, I write fan fiction.
Several things draw me towards fanfic. For one, there are characters out there that I adore and want to spend more time with. Or, there are characters out there that were abused by their writers (I'm looking at YOU, Joss), and deserve a little happy time. Sometimes, I have questions left over from a series or episode. Sometimes, I get an idea that I just can't explore in my own universe.
Mostly, I write Buffy fanfic, but I've also done a few short Star Wars pieces. More than a few of my stories are crossovers - combining one fictional world with another - because the main site I post to is specifically for crossovers. It is a place for me to sharpen my skills - tighten plots, find voice, develop a sense of place and social milieu. And, I'll admit, if you write and post a fanfic that is at all good, you get praised. It means a lot, especially when it's so hard to come by feedback for independent writing projects.
I'd love to hear your thoughts.