Yes, I've written plenty of fan-fiction over the last six years or so. Most of it, especially the earlier stuff is pretty cringe-worthy, if not that bad considering how old I was when I wrote it, but some of the stuff I've written is pretty decent, judging from what other people who've read it have said.
I havn't properly sat down and wrote anything of any length for quite a while now, but this is more due to writer's block than any disinterest in writing something else. I've actually got several rough outlines laid out for Warhammer 40,000 fanfics at the moment, and I'm pretty sure I could make a decent job of it, since my writing skills have definitely improved since I last seriously sat down and wrote anything.
I know a lot of people look down on fanfiction, and it's true a lot of it IS utter rubbish, but there IS also worthwhile stuff out there and I find that writing fanfiction is great practice for moving on and starting your own original works (which I've also got several ideas floating around for, though nothing really solid as of yet).