I'm normally not into original characters in fanfic, either writing them or reading about them. I may use them a tool to propel the rest of the story forward, but not as a co-star (i.e., fully-developed character with as much importance in the story as one of the main characters). That is, until recently - I'm in the middle of writing a multi-part, pre-series story starring an OC who had a relationship with one of the crew. (Oh, man, when I write it that way, she sounds so Sue-ish.) So anyways, since I'm writing a story that I probably wouldn't read if I weren't me, I became curious about other people's takes on OCs.
When I include OCs, they are in the background or they're there for the main character(s). I try not to give them a POV and whatever is going on with them is related to one or more of the main characters.I don't expect the reader to care about the OCs I create within fanfic. I know the reader is there for the main characters, probably one or two in particular. Knowing that keeps me focused, reminds me that the reader will suffer the OCs so long as they further the characterization of one or more of the regulars.I've read some really good fanfics that had OCs who had a voice and a story of their own. It was clearly a balancing act for the writer, finding that line where the story isn't about the OC.
Honestly, it depends. While I prefer a peripheral original character (where they are the minor star of some action, background, or character development), SOME people do an excellent job of creating the co-starring OC through a back-story. THOSE I don't mind reading.That being said, others SIMPLY REEK of Mary-Sue-ism. Physical Perfection, Extraordinary Talent, and The One True Love of "x" character=Mary Sue...whether they really are or not. That's just how I see them & I can't read it anymore once it becomes glaringly obvious--and there are A TON of them out there.
I haven't noticed too many Suefics in Firefly fandom, but maybe I'm just not looking in the "right" places, heh. But in other fandoms, yeah, they are plentiful, especially if there are a lot of young authors.