I am currently on a MLP kick. I've read some Sailor Moon as well, and looked at a few of the more infamous ones (My Immortal, My Inner Life).

For Sailor Moon, the amount of fanfiction I've been exposed to has been very limited. I think I've read stories from less than five authors. There just wasn't that much available back in 1998, or at least I didn't find it at the time. Nothing really impressive, although some of them have disappeared from the internet by now.

For MLP, I tend towards the shorter stories. The problem is that there are a lot of "just okay" stories, and you can easily get six chapters in before realizing that the whole experience is just boring and uninteresting. There have been 15+ chapter stories I've just glanced at and avoided without even reading anything. Plus, the ones that are more interesting to read tend to be the shorter ones anyways; a lot of really long stories are trying to pull a Lord of the Rings, or copy success from one of the longer existing MLP fanfics.