I have read such great reviews about that series though most people seem to feel there aren't enough practice problems in them, though the author feels that once you understand the concepts deep enough, repetition isn't supposed to be drilled in your head over and over. I just bought it and hope to have it in a few days. I bought the fractions one as well. I got it for DD7 (almost 8). It might be a little heavy content wise for her, but, she picks up things so quickly and really loves things like that, that I think she'll enjoy it. Even if she isn't 100% ready for it, I think she'll gain something from it.
