Saylor does his research. Each of his books features an "Author's Note" section at the end in which he details the historical context surrounding the events of the story and lists the sources he used. He may play with the facts very gently here and there, but each of his books is centered around a major event in the Late Roman Republic, and I've always felt that the books provide an incredibly engrossing and authentic portrayal of Rome.

He's also very good at making historical figures feel like fully-fleshed out people -- for example, his book covering the Catilinarian Conspiracy partially inspired me to write my thesis on the subject.