I have used this with my older daughter, and plan to use it with my youngest when she's ready.
I love the program, because each operation is broken down into manageable chunks. My older daughter is a very headstrong type of learner and found this program to be a bit slow, so we've gone back to our other math program. She does, however, very much appreciate that the workbooks are written in a nice simple black-and-white format, as she find many of the brightly-coloured, cartoony books to be distracting.
My younger daughter has some very strong dyslexic tendencies and I think math is going to be a struggle with her. We're still in the very early stages of homeschooling with her (she's 4) but I think the step-by-step program will be a good fit when she reaches grade one.
I'm not sure about the books listed on Amazon, but the books I've purchased in Canada are intended to be a stand-alone curriculum and are used in classrooms. They fulfill all provincial requirements for Ontario and almost everything for the Western provinces.
One thing I did find is that their inherent simplicity could also be a weakness. A good deal of time is spent on each concept, but during that time there is no review of prior concepts. So when we were working on addition and subtraction, my daughter regressed a bit in telling time and counting money. I think if I were to use the curriculum again, I would probably supplement with something like a 'Daily Math' workbook just to keep everything fresh.