I have read quite a bit of fan fiction. While I do read a lot of Ranma ½ fan fiction the majority of it is from other series. While I often point out things I see in the Ranma fandom (mostly fanon), I generally don't feel like writing about or reviewing the other fandom's I monitor. I've actually paid for fan fiction fairly often. Star Trek books, Conan the Barbarian books, Buffy the Vampire Slayer books, etc. Unless they were written by the original writers they are just people writing fan fiction who are lucky enough to get paid for it.
The online fan fiction I regularly read is: Slayers, Kim Possible, Buffy the Vampire Slayer, Star Trek, Teen Titans, Star Trek crossovers, and of course Ranma ½. While there are others I occasionally read those are the ones I read the most often. I'd like to read other series fan fiction but most of the ones that I would like to have too little fan fiction for me to actually follow. Of the free fan fiction, I tend to find the Ranma ½ ones to be written the best, even though they are riddled with fanon and have characters be out of character (the most common OOC probably being that Ranma divulges information on himself and others when he meets new people with the second most common likely being Nabiki's characterization). Lately I've been more interested in crossovers than in universe stories.
I have very little interest in the most popular fan fiction; Naruto and Harry Potter. There are tons of fics for those series but I don't really like the original stories, or more precisely I view them as over hyped, and the vast majority is terrible. The vast majority of all fan fics are bad though (science fiction writer Theodore Sturgeons law “Ninety percent of everything is crap” applies) or much too short. That is the major advantage paid for fan fiction has. The story is generally self contained and complete, though still has rampant with OOC and fanon.
I have read that the majority of fan fiction writers are female (80% according to Wikipedia which was quoting other sources). I wonder why that is? Even series that were intended for males tend to have more female writers than male writers. That said, I have heard that male writers are more likely to write epics (i.e. really long stories) than females are and that the majority of published fiction is also written by males.