It makes me feel curiously stabby.
I think it's a combination of the fact that the sort of people I know who use it are neither the sharpest knives in the drawer, or particularly sincere: it feels like a shorthand that assumes an intimacy that doesn't exist.
Oh, yeah, and it's classic MLM/Poonique-speak, which gives me the rage.
Furthermore, I'm from Glasgow, where the strength and genuineness of your friendship is measured by how cheerily unpleasant you are to one another ('you're a cnt, but you're a good cnt').
The best way to indicate how much you loathe somebody is to be icily polite to them (or call them 'hun', either works).