Good questions.
I see JG Math 5 & 6 covering the same things I covered with my oldest one (pre-Jones Geniuses [JG]) through 8th grade. I used Math U See (MUS) with my oldest and had started the 3 others on MUS. However, after starting with JG, I figured out it got the boys through middle school math faster and more soundly than MUS. The strengths of the JG program is that they get the children doing the basic math operations well. In a normal math program they usually cover addition and subtraction in grades 1-2, then multiplication and division in 3-4, then you move to doing the same operations using decimals and fractions & +/- numbers in grades 5-7 or so. Oh, JG also covers the functions using positive and negative numbers too. Using JG, my 3rd grader is doing all 4 functions in fractions and can handle +/- numbers comfortably. The program was a bit weak on doing multiple digit multiplicaton and multiple digit division, although they did cover it. (Though they cover 2 digit multiplication well.) I like that the boys are not only acquainted with these functions, but are pushed to do them easily and well. For fractions, the younger children are given 20 minutes to do 50 fractions questions. The teen is given 15 minutes. We're still working on perfecting these. Time isn't the problem (they usually have time left over), but they tend to have one or two problems that they got wrong because of a simple calculation error.
But it is a challenge that they are willing to work on. The boys are also learning to do the 4 functions with roots and exponents.
I think the parlor tricks come mostly with finding square roots and cube roots. But they don't stop with just the simple roots that end in 25 and such. They do learn to find the square root of numbers from 2squared to 99squared. And the cubes likewise. In addition to this they get to learn memory pegs for numbers 0 to 100. So, this semester they are memorizing the prime numbers up to 999 (which is not a prime, I understand). This actually came in handy when I was knitting and I needed a number that had several factors.
At the moment, DadDude, I'm not using a different program with the 3 boys other than the JG program. My oldest is in Geometry now and I didn't switch him to JG. He still uses MUS. I sort of like having another math program to look at just to make sure we aren't missing anything. So, I take note as my oldest one goes through his math program and just add it to what I do with my 3 other boys. JG sends a word problem book along with the JG program and that covers some of the holes in the program. The JG program mostly sticks with numbers and working with them. In other math programs they usually cover things like shapes, money, time and measurements (cm, inches, quarts, gallons, liters....). I've covered most of these things with my boys just in normal life situations. I haven't taught them to convert inches to cm or liters to gallons yet, perhaps sometime soon.
It will be interesting to do Math 8. I'll have to order that soon. They are supposed to cover basic algebra and geometry. We'll see if my 9-year-old catches up with his oldest brother.
I hope that is helpful.