I searched for this book for years, and finally broke down and payed the outrageous sum of $30+ for it on Amazon, for a badly used copy, no less.
Was it worth it? Yes and no.
There is no doubt that Evans knows how to tell a story - his descriptions and dialogues flow well, and the story never bogs down. The characters are sympathetic and the major one are well-portrayed.
The negative points:
This is an alternate history, a world where Cortez went over to the Aztecs and helped bootstrap them up the technological ladder. Additionally, a 'New World Plague' akin to smallpox decimates Europe, further helping Aztec survival. However, there's only so many advantages you can give a culture before you enter the realm of the ridiculous. An Aztec Empire that controls more than three quarters of the globe in less than a century of conquest? I just don't see it.
Another sticking point for me was the too-similar similarity to our own world. I mean, this is an alternate earth that diverged from ours in the 15th century - how likely is it that there will be a New York, a Virginia? Or a Canada and New England? Granted, Evans doesn't dwell on _any_ of these cultures with great detail (or even less than great detail), so there's no way to tell exactly how similar or different they are. Even Britain, where the bulk of the story is set (after it is conquered by the Aztecs), is given only a cursory examination of its history.
Aztec culture is explored in greater detail, and the descriptions of the great capital of Tenochtitlan are amazing, but we don't get to Mexico until the last quarter of the book, and it seems rushed.
Another gripe was how the character of Bevan was dealt with. Bevan is a Welshman who becomes the manservant of Britain's Princess Catherine (the book's narrator). It is never clear if he is working with the underground, if he is an Aztec spy, or if he is playing some game of his own. Throughout the story, events seem to be building up to reveal his true nature, but at the end the reader is left as confused as to his identity as he was ten pages in.
So overall, I give Aztec Century 4 stars for the sheer coolness of the idea, but only 2 stars for overall execution.