I am a retired university math professor and a parent of grown gifted children. Many people like Saxon books because they improve test scores and reinforce skills through problems and repetition. They helped our younger "creative gifted" but forgetful son in that respect. Still, I want respectfully to urge that math instruction, especially for gifted kids, not be limited to Saxon.
After years of using them over my objections, apparently to raise test scores, our expensive private school lead math teacher finally told me they were switching away from Saxon because "we found out that after using Saxon the kids didn't understand anything".
From my memory, the books are very mindless rote learning in their approach, symbol pushing as opposed to thinking. I recall they even look ugly on the page, presenting math as a utilitarian subject with no interest or beauty. As I recall one learns how to multiply "X's" together but not what "X" means. Harold Jacobs on the other hand uses empty boxes instead of X's at first, to convey the idea that X is a place holder for other things to be put into. This works so well I have since used it often in college for algebra review.
Even my son who appreciated the repetition, started in glee at the difference between Saxon and the book of Harold Jacobs when I showed it to him. "This is interesting!" (neither of our very bright kids did especially well in math courses in college, at least at first, even though the younger one had higher SAT math scores than mine. The older one, who had won state wide math competitions as a child, went to Stanford where the highly challenging courses bore no resemblance to his high school AP test oriented courses.)
If Saxon is working for you, then use it for its strengths, but I strongly suggest gradually trying to supplement it with more substantive material. Math is about reasoning, not (just) symbol pushing. The fact that Saxon apparently raises standardized test scores is a statement about the level of such tests.
Even the one outstanding math teacher at my university who appreciated Saxon, said Saxon had it right about giving lots of problems and repetition, but that he had it wrong about not giving hard problems.
This is just one man's opinion/experience, but I cannot imagine anyone becoming a mathematician, i.e. learning to love and practice math, from using only Saxon. I feel it may have even helped cause my younger son to lose interest in math.
If you have a really gifted/motivated math prodigy, you might try them out on Elements of Algebra, by the great mathematician Leonhard Euler. With guidance I taught topics from it to some super bright 8-10 year olds last summer before epsilon camp.
http://archive.org/details/elementsalgebra00lagrgoog
