Ratigan, no one can have a higher opinion of you than I have, and I think you’re a slimy, contemptible sewer rat! — The Great Mouse Detective

It never hurts to be honest.  If someone knows just how much you detest them, but can still be professional in your dealings, it will make things smoother.  He may be a son of a bitch, but he might be your kind of son of a bitch, and you can work with your own kind of folks.