This was definitely my favorite of the Riordan books so far because of the strength of the characters. While I think the ending was a bit rushed and Piper became a little too close to becoming a Mary Sue toward the end, I really enjoyed the overall mystery and emotion built into this one. I liked how the characters made decisions not based on foolish childish action but on general belief that what they were doing was right, even knowing they were walking into a trap. I can't fault the original heroes as much for flagging a bit in the early books since they were younger than the new set. But I can say that I enjoyed the new set MUCH more than Annabeth, Percy and Grover. I just think Piper, Jason and Leo are more developed and have more engaging back stories. Even Jason, the classic hero-type, ends up genuinely interesting because of the plot behind him, making up for his more generic qualities. But Piper and Leo, especially Leo, definitely steal the show. The mix of insecurity, personal ticks and angst makes them very compelling characters. 
 I actually didn't miss Percy at all, although I wish Thalia hadn't gotten shafted AGAIN. Why do you hate Thalia, Riordan? That said, I am looking forward to the return of Percy in The Son of Neptune because of the plot that has been promised for him at the end of this one. (view spoiler)[The whole plot with the diaspora between the Greek heroes and the Roman heroes is truly fascinating, especially with Lupa (the wolf mother of Romulus and Remus) as the Chiron for the latter group. I also love watching the different interpretations of the gods, depending on their Greek or Roman form. It's a clever way to introduce the varied perspectives. (hide spoiler)] As an added bonus, I liked how Riordan really went out of his way to showcase the gods who got a bad rap in the previous books, such as Aphrodite and Hera and showed them for their many sides. Having Aphrodite referred to as her "original" goddess self was a nice change from the dippy Aphrodite we got throughout the first series. 
 Really, I have no complaints about this. I might even rank it higher as I think on it. I'm not sure if it's a favorite, but wow it's high up there.