The Lost Hero and The Son of Neptune are both very good although very different but where characters are concerned they both follow the same formula: heroic male lead, slightly-too-perfect-and-somewhat-irritating supporting heroine with a tragic backstory and a well-rounded ethnic supporting male with a dead mum and the coolest powers available.

The Pros:

• The Heroes of Olympus series provides some exposition on gods who had a bad rapport or weren't really important like Aphrodite, Ares/Mars and Hades/Pluto while the gods that were front and center in the original series faded into the background. I like that.
• If you've been following Riordan's books you know that he eats foreshadowing and dramatic irony for breakfast. While still prominent in The Lost Hero, it's somewhat less heavy-handed in The Son of Neptune. Somewhat.
• An exciting change of setting for people looking from a break from Camp Half-Blood.
• If you're a fan of such things you've probably noticed that the books follow the classical Greco-Roman story tradition present in the Odyssey and the Aeneid.
• Romans are cool and the book captures some important archetypes of Roman politics like the stately praetor Reyna or the duplicitous augur Octavian.
• The the next book in the series promises to take the readers beyond the boundaries of North America, and it's about time too.
• The battle scenes are spectacular!

The Cons:

• Some characters (I'm looking at you Piper and Hazel) are not as developed as I'd have like and are sort of annoying.
• This series is aiming to be more dramatic by using a few lesser-known mythological figures to throw the readers for a loop. Even so you may still see a few of the plot twists coming.
• Follows the same formula as the Olympians series. I'm not sure that's a bad thing or not.

And did I mention Romans are cool? Seriously, these guys are hardcore.