The thing about Turtledove is that he's not great with characters (though there are exceptions) but he's positively brilliant at cultures. His characters (even his best ones) have a distressing tendency to be defined by a single trait but his cultures are usually superbly researched and realized. I am truly jealous of his ability to inhabit the mindspace of these people. I suspect that this is only true because of the immense amount of details available which he can absorb since his completely fictitious cultures tend to be defined by a single trait in the same way that his characters are. Here however the true disappointment is that neither his characters nor his cultures feel right. He simply doesn't get Romans. This is a trait shared by many writers, several of whom write Roman novels anyway. The mix of sophisticated organization and primitive structure is apparently a hard one to pull off since it is easy to make them primitive warriors, like Conn Igulden, or else make them too organized and perfect like Turtledove himself does in his Videssos novels and to a lesser degree this book. Apparently Turtledove can do American, Japanese, German, Tudor, Byzantine, and Greek, just not Roman. Which is too bad.
Now I'm just going to say right at the beginning that I didn't hate this book. It filled me with feelings that can at best be described as indifference. It was not a memorable read. I do think that there was a good story in here somewhere, and it does occasionally shine out, but he mishandles it poorly. As previous commentators have said Varus and Arminius are entirely one-dimensional characters. Varus is an arrogant toff while Arminius a reluctant freedom fighter. The plot is also more basic than it could be. There was a lot of politicking going on that could have been explored. The failure to really dig into the characters' motivations leaves the novel drifting, never really catching on when it should. In essence it simply fails to engage. In the end I find I have as little to complain about as I do to praise. And it seems like that's a worse insult to a book than being absolutely furious about it. At least it would have engaged me emotionally.
Turtledove has written some straight historical fiction before now, but he's published it under the pseudonym H.N. Turteltaub. Perhaps he should have kept it for this one. His Hellenic Traders series (Over the Wine-Dark Sea,The Gryphon's Skull,The Sacred Land, and Owls to Athens) is excellent even though they have no real plot. Justinian is the perfect example of exactly what I said earlier: he gets the culture to a T but he can't make his characters three dimensional. Aside from that his bibliography is all either scifi/fantasy or alternate history.