Like most people I have both read and written a few fanfics myself. I have found some that I really like and many more that were just bad.  But, if used correctly it can be a great help for may budding writers to get there feet wet, and explore there style of writing without having to worry about developing characters and a world to place them in.  Most fanfics that I have written where based in the world of the original author but developed my own characters because I am much better a character development than at world development.  I have now started a new story and I am creating my own world, but because I have written some fanfics I feel more comfortable about creating my own place now. But on the flip side many people can become fanatics about what they think is great, and it be nothing but unimaginative crap that no one in the right mind would enjoy reading. These people are the reason many authors do not like fanfics.  It is the few (or many) bad apples that make the world difficult for the rest of us.