My two cents:
I'd never call a girl a cunt because I can see why they would find it offensive. That's not even a word I save for the people that piss me off the most, I just don't use it.
Now my guy friends, I've been known to call them cunts, pussies, etc. just because for some reason it cracks me up. Mind you it's not something I'd call anyone but a guy friend who knows I'm not serious.
I hate the word pussy for some reason and personally would choose cunt over it when talking dirty with the girlfriend, but I'm not really one for having conversation like that with the girlfriend, so it's not normally a problem.
I just go with a nice neutral "special area"