Fan Fiction - Good or Bad?

So between yesterday and today I finished Alice's Adventures in Wonderland (review to follow shortly), and began reading a 'J.K. Rowling approved' Harry Potter fan fiction book titled James Potter and the Hall of Elders' Crossing by G. Norman Lippert. I am only a few pages in, but I wanted to get my thoughts out there on fan fiction. I have never read it before. To be honest, I have never really even heard of it... Is this strange? I'm not really surprised by it, but at the same time I am. There have been quite a few books I have LOVED, and as much as I may have wondered what happened later on, or prior to, I never would have thought to actually write it myself. So, what is your opinion on it? I don't have a lot of time today, so I will just have to leave today's post to this one question to discuss, Fan Fiction: Good Idea or Bad Idea?