Personally, I think it's an insult. But, at the same time, it's just a word. Well, a title perhaps. If a girl is a slut and she knows she is a slut, why would she get offended if somebody called her a slut? I mean, I'm fat, I know I'm fat, so I'm not going to take it as an insult of somebody calls me fat, you know? Why? Because we all know that I'm fat, so it's no secret. If somebody called me fat, they're just telling me something that we all already know.
Anyway, think about this too...
What if the girl LIKES being called a slut? What if it was a turn on for her? Would it still be considered an insult to that girl, even if she enjoyed the title? Food for thought.