There were quite a few twists in this one that surprised me. I'm afraid of spoiling the book, so I'm not going to say much about those twists, but it was a nice change from expecting everything to happen as it really does in the book.
I already know that I'm going to spoil this book a little, so please be aware of that when reading beyond this point.
This one is kind of a second chance romance, although it's not the typical kind. Orpheus is the reincarnation of Skyla's old lover, Cynurus. She'd been sent to kill him then and hadn't done so. I think some of the story gets lost, since Skyla doesn't think about what really happened and Orpheus doesn't remember until the end of the book (he wasn't meant to remember at all). He thinks she betrayed him and gets mad. He nearly does something stupid and then realizes that he does have a conscience (which he curses several times).
I'm not really sure I felt the romance between the two, but they spent a lot of their time traveling or fighting. Orpheus' only real goal is saving his brother, Gryphon from the Underworld and he's more focused on that then any possible relationship between Skyla and himself. Skyla has trouble (at first) seeing that Orpheus isn't really Cynurus. They had sex within a few hours of meeting, but I think their relationship was fueled more by their goals and lust then softer emotions like love. That's why the book lost half a star.
That said, I like this book more than the others in the series so far because of the plot. It was hard to put down. I really liked that Skyla can kick ass too instead of Orpheus having to fight on his own. Skyla being a Siren is confusing, since that seems to be a title more than anything: sirens are Zeus' high level assassins. They are also great singers, like mythical sirens, but aside from that I don't understand why Sirens and not something else? Not really a big deal, but until she calmed someone down by humming, I didn't understand the connection at all.
I'm really looking forward to Gryphon's book, Enslaved.