If Percy Jackson was the perfect hero-protagonist for middle schoolers, Apollo is perfect for the young adults that are somewhat stuck in the no-longer-teenager-not-yet-adult dichotomy. He is self-centered and immature while being dramatic and completely fed up with everything in life in a such hilarious level that is impossible to not find it endearing. And along with the little Meg (that really emphasizes the shift on the target audience: she's the age Percy and Annabeth were in The Lightning Thief, but here she's portrayed more like a kid in comparison to the other 16/17ish characters), they create the perfect formula for both the slightly grown up fans that have been reading Riordan's work since the beginning and the younger, new, readers.
My main complaint about this first instalment is that it felt too much like a first instalment. If not for Apollo's charisma and overall affection for the well-known world, I think I might've gotten bored with the story. It seems to me that The Hidden Oracle's plot is easily the equivalent of the first 30% of Lighting Thief: it is a fun build up, but is, nevertheless, just a build up. And then it ends. It doesn't sit well with me that the first book's whole purpose is just to set up the scene for the next ones.
Hopefully, the next volumes in the series will not suffer from this, nor become responsible for tying all the unfinished plotlines from Heroes of Olympus.
Apollo is the sun god, he deserves to shine on his own.