In "A Game of Thrones" there is a great eunuch, Varys, also called the Spider. He's kinda like the spymaster type and throughout the books its hard to tell what side he's on. Also, there are mercenaries that are castrated and trained from boyhood called "the Unsullied", very loyaly troops.
Also, there is one in the Thieve's World set of books, forget his name, but he owns a scriptorium, also another schemer-type.
I would think that castration would give a automatic bonus to Constitution, as eunuchs develop large bellies and man-tits.