Many people say that fan fiction is simply a copyright infringment and should be dealt with harshly. Still, others say it's a valued tool in their arsenal. I can't speak for everyone, but I do know what I think.
I've written original pieces for years, and fan fiction right along with it. I currently have not one but two full pages on two different fandoms and a smattering of other stuff elsewhere. When I'm bogged down by plots and character problems, or hit by that dreaded thing called writers block, I have an escape. I can simply go to another file, another world and write. I don't have to pick at every little detail, I can simple throw the characters around and let my muse flow.
It recharges those creative batteries that we all have and I know for a fact that we need them, I need them. In the time I've written fan fiction I've learned all about plots, about how to build a character from nothing. I've also created a support network of other like minded people that is frankly astounding.
When I went to both groups I'm associated with and said, "I'm writing an original historical romance I'm going to try and get published" the outpouring of support and well wishes was astounding. It humbled me to realize that in the years that I'd been writing and posting what so many discount as unimportant crap, I'd been building friends along the way.
You may have an opinion on fan fiction, like it or dislike it. Think its valid or invalid it doesn't matter to me. Either way, I believe quite strongly that Fan Fiction is a tool - and the men and women that write it are as valued and creative as those who write original pieces and have never tried it!