I think this work really shows how far Riordan has come from his early Percy Jackson days. Your villains are no long unsympathetic and your characters are intelligent and don't just luck into their victories (most of the time). I also really enjoyed both the American history (the Civil War) and the Roman/Greek history (the assimilation of Greece) that was weaved throughout this book. I really felt like I was learning some things, just as the characters were. Many of the characters also had amazing development in this book, that banked off previous books in this series and the entire Olympian saga. It was fascinating to see the gods having trouble balancing their Roman and Greek incarnations and how that affected them. In general, I really enjoy the way Riordan plays with this concept, which is something I have not seen in other Greek/Roman myth stories, actually showing how it wears on the gods. The villains were also just fun, which I think was missing from the first two books, where the villains were just evil. The action scenes were great, the puzzle scenes were great and the descriptions of Rome and ancient Rome were also amazing. 
 My greatest nitpick with this book is the intense attention paid to romance and how this resulted in an abundance of love triangles, to the point where characters at the center of these triangles ((view spoiler)[Hazel (hide spoiler)]) were not allowed to develop as characters. I also had issues with the way Reyna's character was reduced far too often to being the one who lost in love. I am indifferent to the Percy/Annabeth pairing and much of the way Riordan pushed their romance brought back too many flashbacks of J.K. Rowling's attempts with romance in The Half-Blood Prince. Too much at once. That said, things cooled down toward the end with all the respective romances. I also whine that neither Frank or Hazel had any chances to really shine in this book. 
 My hopes for the next book are POVs from Jason, Hazel, Frank and (view spoiler)[Nico (hide spoiler)], less focus on love triangles and more moments of awesome for the characters who got shafted in this book.