Eh, as far as the -er form goes, I avoid saying it, or indeed thinking it, and I've been known to pull up friends thats use it; it just seems to have inevitably dergotative connotations, and certainly they've always used it as an insult. The -a ending seems to have a whole different meaning, a whole different 'feel' to it, I guess. I don't know many black people tbh, but if I did, I suppose I might fall into using it amongst friends, and that's probably not a problem.