I rarely read fanfiction stories, but not because I have any kind of downer on them. I'm just often not familiar with the original work - for example, I've never read a Harry Potter book, nor seen a Harry Potter movie - and clearly to get the most out of the fan stories you need to be familiar with the source material.
I can definitely see the appeal, though. You have a ready-made cast of characters, so can get straight to the action without worrying about a lot of exposition, but you still have the challenge of retaining the truth of those characters - and fans will always be intrigued to see what you do with their favourite fantasy universe.
It can be a route to big things as well - I gather E L James started off writing Twilight fanfiction. Opinion is divided, of course, as to whether she should have stuck to it.