I've used Shiller Math for four children:
DS, now 19yo, homeschooled through 8th grade. ShillerMath was great for him through 6th grade, which is when we finished the program. He spent 7th grade filling in some gaps (practice with long division, practice with fractions, others I can't remember) with Math Mammoth Blue Series, then did Foerster's Algebra with Math Without Borders in 8th grade. Went to brick&mortar high school (Geometry, Algebra II/Trig, Calc AB and BC); now he is a math major in college.
DD, 15yo, used ShillerMath for pre-K through 3rd grade. At that point, she switched to Math Mammoth Light Blue Series because she likes orderly worksheets and doesn't mind working on her own. After Math Mammoth 7, she started AoPS Algebra. Took online Algebra I through AoPS, and online Algebra II through WTMA.
DS, 11yo, still using Shiller Math. We just finished the Fractions book. We are moving slowly through the books because I'm not prioritizing it and we also play a lot of math games.
DD, 6yo, is working in Shiller Math books 1 and 2.
My overall assessment of Shiller Math is that it is excellent in teaching math concepts. It is also fun, at least for me and my kids, and I think my kids have learned to love math because of it. It is weaker in drilling facts. However, there are drill lessons, such as bouncing a ball back and forth while quizzing facts, etc. We just circled back to those lessons over and over. With Shiller Math, you can't have the mindset that one lesson = one day. We might do 10 lessons in a day, or might take 10 days to do one lesson over and over (for example, beginning 4-digit addition with DD6; we did it over and over because she wanted to, not because she didn't "get" it.) There is one bizarre hole in Shiller Math, which is that they don't teach the typical method for long division - although with the conceptual process that they use, the typical method is easy to learn. All in all, I think the combination of Shiller Math and Math Mammoth is fabulous. I would wait until a child was in about 6th grade to do Math Mammoth. I also think you could possibly go straight from Shiller to AoPS PreAlgebra.