It's lavishly written and directed, as well as historically engaging, well pitched and for the most part is spot on with its depictions. They've also managed to insert the stories of Pullo and Vorenus quite expertly, making them interesting characters, without being too offensive to the historical veracity of the series. On the whole it's superbly acted, adding genuine human drama to perplexing and exciting events.
The lives of Pullo and Vorenus occasionally err into less exciting territory (I found it hard to care about Vorenus's children in series 2, but I understand why they were necessary to characterise him, especially in that series). The females add a soap-opera quality to the proceedings, but given what we know of some Late Republican women, it's not entirely unwarranted. The female Claudii and Sempronii, I am thinking of particuarly. Marcus Crassus is nowhere to be seen, and despite dying in 53, gets no mention (although his son does, fleetingly). Marcus Agrippa is poorly constructed characterwise, for the depiction seems to be nothing like the robust Agrippa of our sources.
On the whole though, I do love it.