The following is about the series as a whole, to most of which I give 5 stars. I actually liked the later ones better than this one, the first, which was certainly good.
I'm an amateur Latinist with scholarly training, and have long had a particular interest in the late Roman Republic, the period of this series. I'm astonished at how well Roberts gets into the minds of the Romans, into the details of their lives, into events and the politics and other factors behind those events. I find them much more true-to-life than the other Roman series (Saylor, Davis, which I nonetheless liked). This is the real Rome, not Hollywood Rome, or the Rome of second-rate teachers and innumerable hack historians. As far as large-scale events, their causes, and implications are concerned, you can just about read these books as history. Where the history is subject to interpretation, Roberts is among the most perceptive interpreters. In fact, never mind the "just about". Many historians have been less adequate and less and balanced.
Even the historical characters seem astonishingly real, and true to history and to the personae they have left in literature. Roberts makes even the shadowy Pompey into a lifelike character as has no other historian I've read (I haven't read Gelzer on P.), a character who would have done exactly what the historical Pompey did.
One notable exception (and virtually the only significant departure from known fact that I've detected) is the character of Milo. Roberts' Milo is plausible enough. He's a bit larger than life, but this was a setting (like Elizabethan England and the early American West) that generated many people who actually were larger than life normally gets. The real Milo did what Roberts' Milo did in this series, as far as the historical record shows. But I've never seen any indication that Milo was ever a free rower, or that his origins were in the street. The historical Milo was from the local nobility of a town outside of Rome, though he was certainly a gang-leader and political agitator in Rome, and nothing much else. If I recall correctly, in later books, Roberts reconciles Milo's background with the historical record as far as possible without deviating from the account given in the earlier books. I would guess that Roberts' Milo makes for much better fiction than the real one would have, and again, this is the only departure from history that I've detected, and one which does not entail any other departures.
The historical accuracy comes with no tradeoff in entertainment value, however. Roberts is a fiction writer first and foremost. Characterization, color, action, plot, and puzzle all make for a first-rate read.