I haven't actually cosplayed an original character yet, but I will eventually. I think it would be fun, and I myself love seeing original cosplays. They can be so creative and interesting! (:
However I don't think that I would ever cosplay an original character at a con, because no one would know who I am cosplaying as.
But I also like the idea of designing your own original variation of a certain character (I'm actually working on some now...)
And I think if that character is known well, then it would be great to wear it at a con! ^_^
Currently in the making:
Planned (within a year):
Steampunk Kairi (Kingdom Hearts II)
Daenerys Targaryen (Game of Thrones)