Not all ad-hominem arguments are faulty logic. This article discusses when to use them appropriately. This article is based on a book, called Media Argumentation: Dialectic, Persuasion and Rhetoric, by Douglas Walton which can also be found here. The short of it is, that if it is really true that, say, for instance, someone were to say “he is totally unqualified to be President because he had the attention span of a gnat on amphetamines and can’t read,” it would be appropriate because it is both true and relevant to the job.
A new theory parses fair from unfair uses of personal criticism in rhetoric