I would recommend this book for boys aged 8-11. This is definitely not for young adults (I think they'd be bored), and it's not easily enjoyed by adults unless they have boys in the target audience age range.
The book starts out with an action scene that is quite intriguing. I liked the brief introduction to the opposing forces in Fermicia. After this first action-packed chapter the next third of the book is slow and awkward. The introduction to Jake and his first day of jr. high is dry. The glimpses of the changelings being the only redeeming factor.
Once the main characters all made it to Fermicia, things picked up. Though it seemed a little too easy for Jake to just leave home with barely any explanation as to why he was going and what was in it for him. He wasn't given enough motivation to leave home on an adventure from which he may not return. Joraus guilted him into going, and that's really not a theme (peer pressure, bullying) that I would want to convey as all right to this age group.
We're introduced to the other main character, Sam, upon arrival in Fermicia. She seems quite a bit older than Jake. She has a sad family history which may have helped her grow up fast, but she seems several years older than Jake's 12. Unfortunately, this main character is hardly developed in the story. She feels more like the outline of a character than her own person.
The children progress through their adventure and we seem to lose sight of the fact that they are 12. They're called children often enough, but they're required to make very adult decisions and take very adult actions. I find it a bit appalling that the changelings require Jake and Sam to kill their enemies without a second thought and never seem to have time for any sorrow or loss of innocence. This, in my opinion was a huge failing. It could definitely have been handled more delicately and allowed for awesome character development. Missed opportunity.
I was also disappointed in how easy Ragesong was for the children. They grappled with it some, but I would have liked to have seen more of the struggle. Jake goes from not being able to touch it at all to becoming one of the most fearsome fighters in all of Kardonin. I get that they are talented, and actually loved that their musical abilities were tied to their fighting abilities, but I wanted more growth.
I saw the ending coming way before it did. I wasn't disappointed with it, but it was a little cliche. Unfortunately, I'm not exactly curious to see what happens in subsequent installments.
Overall, I'm not sorry that I read this, but it wasn't the best children's fantasy I've come across. Young boys will probably love this for the action and adventure. Girls will probably have a harder time relating.