This was a rec from someone, and I have to say, I truly enjoyed it and read it in the course of one day. Being an Alexander fan along the lines of Mary Renault it was a pleasure to read a sympathetic characterization of him. Plus, Hephaistion gets the place he deserves - by Alexander's side. :)
I'm also a big fan of alternate histories, so I enjoyed this aspect of the book as well. Alexander fighting the Romans was especially interesting, I thought, as were the tidbits of "future" that showed how history might have changed if he had succeeded in creating a stable empire. As far as I was concerned, the book could have gone on much longer!
There was so much detail in everything, from the battles to the "future" stories, I easily forgave the slightly uneven writing and the few facts that were changed before the point of divergence and left unexplained (i.e. the Sacred Band and Alexander's son). I do think already having some background knowledge of Alexander's life is helpful in order to fully appreciate the novel. But yes, definitely a pleasant surprise!