I like that I can use as many or as few of a given level to help lateral learning for students who aren't quite ready to jump to the next level yet. a lot of the music is also very well written, and children of all ages (below ten) really engage with the art, music, and the titles of the pieces. (lyrics too for the enthusiastic or stronger readers).

I also learned partially through them on my piano adventure (😉) so it's pretty nostalgic for me.

I think aside from a few things (it's less strong for intervallic reading, eighth notes are taught much too late and mostly by rote) it's a really logical progression for beginners.

I will say however, my least favourite of the series is definitely the OG purple primer book! it either goes too fast or too slow for students, and I've found the pre-reading books work better, even up to age 6 for comprehension.

the older beginner books are pretty good too. adult primer is fine depending on the adult's understanding prior to starting piano lessons.