The average rating is going to depend on a few variables. It will depend on who you ask more than anything else, but the number of reviews is important too. A small sample size is going to skew the result more one way than another.
I checked the average review from Amazon books. It's average reader rating is 4.5 out of 5. It is based on 793 reviews. That's a fairly large sample size, with consistently high ratings. Barnes and Noble gives it the same review score, and it is based on 298 reviews. That's just about 1100 review ratings given with an average of 4.5. That's really good.
Both of those sources classify the book as children's literature or kids' fiction. Scholastic.com recommends the book for 6-8 graders, and the suggested reading level is grade 5.5 or higher. The targeted audience is junior high students. The main characters are male, so the book is geared more toward boys than girls.