To be honest, I don't really have much of an opinion on fan fiction. I don't actually have much of an opinion on people using my characters in fan fiction. For that matter I barely have an opinion on "slash" fiction (although I still find the idea of Good Omens slash fiction fairly mindboggling) (er, and Knight Rider slash fiction. I think that Knight Rider slash fiction is pretty weird, to be honest).
As long as people aren't commercially exploiting characters I've created, and are doing it for each other, I don't see that there's any harm in it, and given how much people enjoy it, it's obviously doing some good. It doesn't bother me. (I can imagine a time and circumstances in which it might. But it doesn't.)
Either way, it's a good place to write while you've still got training wheels on - someone else's character or worlds. I remember, as a nine-year-old, writing a Conan-meets-some-Ken-Bulmer-sword-and-sorcery-characters. And it's fun to head over into someone else's playground: I've written several stories over the years set in other people's worlds (including an episode of Babylon 5); and if I don't miss the deadline, I'm meant to be writing a Sherlock-Holmes-meets-the-Chulhu-mythos story very soon.
I do understand that there are grey areas, and I think of fan fiction as existing in them. I know authors who love fan fiction based on their stuff. I know authors who have formally attempted to stamp it out. I'm just sort of [shrug] about it.
I don't honestly mind if you stick (for example) Shadow or the Marquis De Carabas into a story intended for your friends, and not for commercial exploitation. I'd rather you put a note at the end saying who the characters belonged to, which most fan fiction people seem pretty good about doing anyway. But I'd hope you'd see it as a privilege and not a right.
(On a similar subject: Every now and then someone wins a local short story competition using a story or plot of mine, and I hear about it (often when they send me embarrassed notes, years later) and I try not to grin, and to look angry, but I haven't managed it yet. I keep meaning to tell Marv Wolfman that I won a school essay competition when I was twelve with a horror-comic plot of his....)