I was ready to give this three stars, but the climax saved it from being just okay. Riordan does know how to write action and introduce a well-earned plot twist, so I bumped this up to four stars. It also helped that Apollo stopped patting himself on the back just for existing, because the dude was annoying me in the first half. It's good he's going to get actual character development and not just be a one-note arrogant tool.
I've only read the Percy Jackson series before this, and while I enjoyed those books, I ended them figuring I wouldn't read anything else as they started getting samey. And much of this book is also samey, but eventually the characters, especially Meg, won me over. I wasn't lost at all while reading it, since Riordan gives sufficient background into the events from previous books. It was good to see Percy, Nico, Rachel and Chiron again, and I'm interested to see how things develop between Apollo and Meg, and how or if Apollo is able to redeem himself and how certain oaths he makes are going to shake out.