Signed DD (just turned 9) for the Art of Problem Solving pre-algebra I course and have been working alongside DD on the prep for lesson one with the book to get her used to the new format (finished SM5 a year ago).
I have to say that I wish this had been around when I was a kid! Really gets the reader thinking about the most efficient and therefore 'easiest to compute' (my quotes) solutions from the first page!
Here is a quote from RR on the genesis of AoPS books that I will share because I am sure that it will resonate with many here...
We started the Mandelbrot competition at the end of my first year of college and we ran into a little bit of a problem with the competition because the tests were really hard and the scores were pretty low. We had to make a decision as to whether to dumb down the tests or try to teach the kids. We did something that’s a little novel in American education -- we opted to teach the kids. That’s where this started.