Harris has done something really smart here: if he'd published a three-volume biography of Cicero, no one would have read it. (Well, I wouldn't have.) So instead it's a trilogy of historical novels, which sounds way more fun. But it comes down to nearly the same thing, right? This is a very detailed, carefully researched work about Cicero.
It's told first-person by Tiro, Cicero's scribe, who's a real guy who wrote a real biography of Cicero (now lost). It's a clever gambit by Harris; it allows him, among other things, to slyly inform you when the passage you've just read is the actual transcript of Cicero's speech, which happens often. He just has Tiro say something like, "And I am certain that the above speech is exactly as he told it, because I wrote it down myself and the record still survives." That sentence is exactly true.
Harris's writing isn't always the most graceful - he's better on plot than style - but he's not incompetent. Here's a great passage describing Cicero:
"I pictured his quick thoughts running ahead in the way that water runs along the cracks in a tiled floor - first onward, and then spreading to either side, blocked in one spot, advancing in another, widening and branching out."
That's a very cool image.
For the most part, at least, Harris's research is meticulous. The first half of the book culminates with the trial of Verres, which (I looked it up) happened exactly as Harris writes it.
Unfortunately, Harris seems to abandon historical authenticity toward the end of Imperium; without giving too much away, the intrigue behind the climactic race for consulship is completely invented. To my knowledge, at least. I'd love to be proven wrong here. I researched it as best I could, and it looks like the behind-the-scenes maneuvering was over other things entirely.
Still, I liked it enough that I'm totally looking forward to the second book in the trilogy.