I kinda like it. Not enough to attempt to read every fanfic ever created for a topic I'm interested in, but if it's a well-written story I'll take a look. True, it's not as creative as a truly original story would be, but you'd be surprised how quickly a fanfic can spin off into its own entity that eventually has little resemblance to the original concept.
I wrote one myself once, a fanfic of the Ozy and Millie webcomic, though that was for a school project and I didn't have time to come up with an original concept. I'll admit that while I wrote it, I couldn't shake the feeling that I was ripping off the guy's work and stealing his creativity, but meh, it got a 100, so whatever. I guess part of the reason is that I wanted to keep them as in-character as possible, yet doing that made it feel almost like plagarism, despite the fact that I only used a few direct allusions to the comic. Regardless, it's not something I'd probably do as part of a habit, though I think I might do it again if I could come up with an original enough story that still made sense in the Ozy and Millie universe.