This is the first book by this author I have read. It lacks action. The structure of the book is such that it inevitably focusses on one disastrous battle but before that little actually happens. The book isn't just a simple 'swords and sandals' novel of Roman army action. The author has tried hard to examine the motives of the protagonist and antagonist in the overarching history and he acknowledges that his book is a novel and not a history book. Accepting that, the writing is good, the dialogue appropriate to each speaker and the feeling of the times comes through well. I liked both the descriptive prose and the narrative prose as well.
The barbarians IMHO were well portrayed and the rigidity of the Romans is also displayed well. Arminius' motivations are well described, probably not as accurately as is possible but it is a novel not a history. I also think the depiction of Varus was a little too kind. Robert Graves gives a very good account of the entire episode in a few pages of I, Claudius so if you didn't like this book maybe you should just read that.
The problem here is that we only have the Roman's account of what happened. Turtledove describes torture by the Germans with a delicate brush and I think he does that well - it isn't easy to describe torture without being horribly gory.
We only have the Roman version of the whole episode and I suspect it is heavily coloured by the Roman wish to demonise the Germans afterwards. The Germanic people were not as backward as many believe - they had a well-structured society, a strong moral code and deeply held religious convictions.
My over all impression was that it was good but not tremendous. It isn't as strong as Scarrow, Iggulden, Doherty, Turney or (not quite in genre) Cornwell.
Despite the lack of action making it hard to get into, I did enjoy the book.