The thing with these books is that I always end up reading them really fast because it's very hard to put them down. Yes, the style might seem "childish", but it doesn't bother me one bit. The use of mythology is really interesting and I quite like the take the author takes on some myths. Percy Jackson may not be the smartest of heroes, but I don't find that as frustrating as some. 
 In terms of the new characters, I quite liked Thalia and her struggles, there is great potential for a deeply tragic character here and I think the author handles that well. Same thing with the di Angelo siblings. I'm not quite sure about the way the Hunters are portrayed like some very negative thing, but Zoe was awesome enough that it made me forget it. Plus, that's the problem when you have an internal POV : it's limited, and the view given by the book will always be the character's views, with their flaws. 
 I'd say I'll be reading the 4th book very soon. Very soon indeed.