I think I have a second hand copy of the Thorn Birds somewhere, but never got around to reading it...
Agreed. For me it pretty much defines my view of that whole period, especially the earlier part which is a bit less often depicted in popular culture - I'm still inclined to view Sulla and Marius the way she depicted them. Caesar, Antony, Augustus - there I guess it's more a mixture of sources.
It's rather didactic that way - focusing almost exclusively on the big historical characters and going out of its way to cover almost everything of historical importance happening over the period, you could say it's more a fictionalized, glamourized retelling of history (the old kind of history, with heroes and villains determining the course of history rather than socio-economic developments) than historical fiction in the normal sense. But that's not a bad thing.