It can be good, but it has a great tendency to not be good.
It does exist, but you really wonder why it should exist.
You wonder why any people would try to emulate another person's story, and then you see that it is strikingly popular.
It can be deep and insightful, but that's just not what you want.
It's usually sloppy and always in need of an editor, then you see those who are better at editing that it's pretty presentable.
It's usually short and oft unfinished, then sometimes there are ones that don't give up the ship.
Heh. I can't talk down on it because there are always exceptions to the broad flaws. Hell, that is the entire description of why people read fanfics: There is always a good in the group of bads. It just depends on so freaking many factors (how much you like the base material, the audience the fandom attracts, the author, the focus of the stories and canon connections, general writing elements) that it really is a bigoted decree that all fanfiction is bad.
If you don't like fanfiction, obviously, you don't want the base source to be expanded on or you just haven't found the correct author. All fanfiction communities suffer by being a grand potpourri of genres written by a grand potpourri of editorless wordsmiths. The comedies are mixed with the fluff, which is mixed with the epics, which is mixed with the OCs, which is mixed with the world-building, which is mixed with the prequels. And the writers - who the fuck is going to stop them from butchering mingling genres besides their own sense?
Sometimes it's worth it.