I've not cosplayed an original character but I really want to! I think it's a great way to unleash your creative side ^.^
When I'm at cons if I see a costume I like i'll go and tell that person, and maybe get a picture. I don't know as much anime and manga as alot of people so chances are i'm not gonna know if something is OC or not, but to me it doesn't matter. An awesome costume is an awesome costume!