A small portion is damn good. A large portion is forgettable. A small portion is goddamn awful. I trust that you are capable of forming your own opinion on the matter.
When you read some good stuff, check out that author, and any other authors they recommend.
Also, it is free.*
The first really good piece of fanfic I enjoyed was "Kid Dynamo". It featured a new character joining an existing group, but avoided a lot of Mary-Sue-isms, and was just an enjoyable read. Took the existing characters in some interesting directions, had some great turns of speech, entertaining plot twists, and has a permanent place on my hard drive.
Don't know who said it, but "Fanfiction is a culture's response when it's mythology is owned by corporations."
*Internet access required, which may require additional fees, and/or ownership of Internet capable hardware.