It depends on the country and culture too. Here in Scotland, swearing is basically part of the vernacular, especially if you're in informal terms with each other. For example, if I help out a friend, he might say "ta, you're a good cunt!", and it's a way of saying "thanks".
It still has negative connotations, but in a much lesser way so it becomes part of friendly banter.
Another example would be if I told a friend something incredulous, they might respond with "get tae fuck!" (meaning, "fuck off!"), or "awa' an shite!", (meaning "what a load of shit!").
Because there would be violence involved.
Seriously though, the tone, context and body language would be completely different.
Too much familiarity with someone you don't known is a highly aggressive challenge. "What's up motherfucker" to some guy at the bar won't make any friends and might end in a fight.
I'd rather my friends call me a cunt (as the closest ones do) than mate, because the thin veneer of politeness in mate implies distance if not contempt (from strangers / acquaintances).
No it isn't. It's often hilarious, as in the above example. And in that one episode of Curb Your Enthusiasm. You wouldn't use it in mixed company, but if you know your audience you can say any number of horribly offensive things.