Having recently read all the Falco series, I would say that this novel is similar but there are several differences which may put off fans of those books.
The main one is that this story is written in the third person as opposed to the first. There is still a fair share of cynicism and sarcasm but you don't get the snappy asides and put downs that occur regularly throughout MDF's trials and tribulations.
It is not a detective story either although one of the main characters is an investigator at various points in the book.
I also didn't feel it had quite as much humour as the Falco series but being set in the rather darker period of Domitian's rule as opposed to Vespasian's this is appropriate.
There are several minor characters from the Falco books who turn up here or are referred to but there isn't any real continuity from that series. This is definitely a stand alone novel so do not worry if you haven't read the Falco books. You won't have missed anything important to the plot and if you go on to read the Falco series later, I don't believe I spotted much that would ruin any of those plots either.
Several people have commented on the scene with the fly. I'm not really sure what to make of it myself. It just seemed odd and there was nothing similar in the rest of the book.
There are some large chunks without dialogue. This didn't bother me but your mileage may vary.
Concerning authenticity and whether certain words or phrases would have been in use at the time: I spotted what I think are a couple of examples where this seemed to be the case but I'm not a scholar of Roman history so this is just a feeling. Nothing particularly jarred though.
One point I would make, particularly if you are new to Lindsey Davis books is that there is a certain amount of swearing - some of it quite strong. I wouldn't call it prolific but if you, or someone you are buying this for, do not appreciate the F word several times, you may want to avoid.
There are also several conscious or unconscious nods to Rosemary Sutcliffe, who wrote the Eagle of the Ninth and several other novels about Roman Britain. The most obvious being the inclusion of a song from The Eagle (and a new bawdier version of it as well). This is referenced directly in an addendum. Also, one of the main characters having serious wounds or a disability, which is a common theme in the Sutcliffe books.
Overall, I enjoyed the book. I didn't find it too long. My interest didn't flag and unlike some, I found the seperate themes of Domitian's descent into paranoia and the developing love story between Vinius and Lucilla to work well together. I've knocked one star off because before I would have preferred another Falco novel but this is still well worth reading.