This book combines two qualities that I find essential in a history work: It is extensively, indeed exhaustively researched, and it is eminently readable and accessible.
I have been primarily a student of Roman/Byzantine history, while naturally developing a fair amount of knowledge about the history of the Gauls/Franks, Persians, Carthaginians, and Persians, among others. I know Hannibal because I know Fabian (and Scipio), Vercengetorix because I know Caesar, and so on, but I knew little about Mithradates prior to reading this work. I was particularly interested to learn that Mithradates was a historical character of considerable fame throughout the middle ages and renaissance. While I have of course previously read of the campaigns of Sulla and Pompey in Asia, this had always been from the Roman point of view, with little effort to provide insight into Mithradates, their primary opponent, and his realm. Apparently the old boy has fallen out of fashion for a hundred years or so.
As I read "The Poison King", I found myself constantly amazed at the wealth, activity, cultures, and leadership in Pontus. While many of the detailed records of his life are lost or colored by their Roman filter, Mithradates remains a compelling and fascinating character based upon what we know and may reasonably infer or surmise. It is surely not overstatement to say that he was Rome's most feared enemy for fifty years. If you are interested in the Eastern theater of Rome's empire prior to the fall of the Republic, I think you have to regard this as a must-read.
Regarding some of the negative reviews: I almost have believe we didn't read the same book. I read the work with care, after reading at least two negative reviews here, and keeping their negative commentary in mind I found no merit in their views. I'd say someone has an axe to grind, or is sufficiently concerned with modern politics to be unable to discern a truly objective, scholarly, and entertaining work of ancient history. I will say that this is primarily a work of history, not merely military history.