I have used it for my 6 year old (along with Singapore Math and Miquon). It was worth it for us because my son derived a lot pleasure from it (he even has Math Monster clothes that he wears often because he loves the "beasts" in the series very much).
But, I have heard that some kids found the comic book format very "busy" and distracting.
My suggestion is to print out the samples and show your son. If he likes it, just buy the 3A Guide and practice books and try them. You can decide how to proceed from there.
