Surveys in Britain generally say that it's the swearword that makes more people cringe and look away than any other - though racial epithets have probably overtaken its position in recent years. Still, it's only 'affectionate' in the sense that any other extreme swearword can be affectionate between two friends. (Black Americans calling each other 'nigger' springs to mind here.)

I do think ther is a difference though: in American English, in my experience, 'cunt' seems to be regarded as a specifically gendered insult. It's only directed at women, and seems to carry the meaning "you're worthless apart from your genitalia" or something like that. In Britain, it's become separated more from its literal meaning, and simply carried the connotation "utter bastard" or "really hateful person". In fact, while in the US I gather it's considered a demeaning insult, in Britain it's more often levelled against powerful people who are abusing their authority. The policeman who gives you a speeding ticket when you were hardly over the limit is a cunt; the referee who awards a penalty against your team when anybody could see the other guy took a dive is a cunt.