There is a lot of terrible fan fiction out there, but there are also a large number of real gems - at least in my experience. Granted, most of my experience with fanfiction has been with anime (mostly Ranma) and Harry Potter fanfiction. Perhaps one out of 30-40 stories was worth reading, but you can usually tell after a page or two whether or not it is worth continuing reading. Also, it helps to frequent websites that require one or more betas (from the site's pool of such) before any chapter is posted. The overall quality - in writing and sometimes even in plot - is usually better at such sites. Finding such sites, however, is not always easy, as they are often highly focused in the types of stories they accept (certain pairings, certain styles, etc).

So, I suppose I have mixed views on fanfiction. Some, I really wish I had not read even a single page, while others I consider myself lucky indeed to have stumbled upon. The author's skill and whether betas are used can really make a difference, I think.

While I myself am not an author, I can see how fanfiction could be used as a means of practicing one's writing, finding one's style, etc. In fact, one of the best fanfic authors I have ever read is also an aspiring author, so I suppose writing fanfiction has likely aided him, at least, in finding and improving his style of writing.