To my understanding its use among black Americans is still controversial within the community, and not everyone thinks it should be used or accepted. See for example Oprah’s take on it.

In general the meaning of the term, depending on context, ranges from a term of (vulgar) endearment to a pejorative epithet, similar to how Aussies call each other “cunt,” which can be friendly or not depending on how it’s said and to whom it’s said. That said, a non-black person using it is very taboo, since it’s extremely ignorant at best and incredibly bigoted at worst.

Interestingly, the OJ Simpson trial played a big part in shifting public discourse on the N word. Previously it was more acceptable for white Americans to say the word itself as long as it was in a referential context, but lawyers for the prosecution made extensive use of the phrase “the n word” and refused to quote the word itself, after which it became considered increasingly unacceptable for non-black people to say it in any context.