My son (11) uses the subject books to review concepts when we have a few weeks in between starting a new Life of Fred book. There is a lot of repetition. I go through the assignment and only assign the odds or evens. He really does not like repetition. The books are very self-explanatory with clear and concise directions.
This is our core math curriculum and it has been a great way for my son to learn. He picks up math quickly and this was a great way for him to learn more independently while I was working with my daughter. The books are inexpensive and you can get them printed and bound relatively cheaply. We use the books by grade level, although I have used the subject books when we need review on a single subject.
The only issue I have had is they have a lot of repetition. This is great for kids who struggle, but for those who catch on quickly it can get tiring to do all of the problems. To work around this I would have my son show mastery and then skip the problems he understood.