What's even more exciting is that, in true Fred fashion, they aren't just getting math concepts but biology and economics thrown in there. For example, the first book includes the following topics:
seeds, plants, kingdoms, genes, bones, skin, DNA, volume, diameter and circumference of a circle, sets and negative numbers
just to name a few of the topics. It looks like the book does a wonderful job of covering biology for the middle grade students.
In the second book, many timely economic topics are covered as well as math concepts. Some of the topics covered in pre-algebra 2 include:
domain and co-domain, how to run a successful business, money, capitalism, socialism and communism, loans, freedom vs liberty and Venn diagrams.
I will be starting the first pre-algebra book with my son who is entering 7th grade this year. Depending on whether you student finishes fractions and decimals/percents in one year or not, you could be starting this in 6th grade, which I think would also be appropriate.
If you are looking for an interesting middle-school math program, I think these books fit the bill. You can get more detailed information and order the books from my website, Catholic Homeschooling Resources.