Fanfic, unfortunately, usually isn't good. I can't stand fics with massive grammatical, canonical, etc. type errors, and I foam at the mouth when characters are blatantly OOC or Mary Sue. This, sadly, rules most of it out, but I do have two or three authors that I keep going back to in various fandoms. When it IS good, I will admit it, and read it, assuming it is also full of smut.
Granted, I've also written it (in a veeery small fandom), and I am a beta for several other major ficcers. Fanfic is a lovely break from "actual writing" because it allows the author to take a break from pesky things like plot and focus on character interaction and development. That makes it fun! It's like building a puzzle in a lot of ways, and the end result can be very pretty to look at. Since I also write erotica, fanfic is a good way to relax and take a break, but still keep in the habit of writing.
Bottom line, I think that no one knows characters better than their original authors, but a good, smut-filled fic can still be a lot of fun to read.