I've never used it, but I just looked at it and have considered it before. The series is not repertoire, but a collection of supplements on each level, so everything is in one book. The Piano Lesson series is scales, theory, vocabulary, and music appreciation. The Piano World is a faster moving series, with Contract for Piano Lessons, Assignment Sheets, Warm-Up Exercises:
Relaxation Exercises, 10 Gearing Up Exercises; Scales:
River of Scales,C Major plus sharp key Majors in five-finger patterns: each written hands separately;Theory:Continent of Theory,15 lessons include:note identification, musical alphabet, drawing notes, music math, Treble & Bass Clef lines and spaces; Vocabulary:Piano Language: 15 terms and definitions; Music Appreciation:Piano Culture-Opening new doors 10 Doors include: Meet your piano, clavichord, harpsichord, Christofori, Toy Symphony and more; Expedition Report: a synopsis of data to be recorded for over the 15 weeks.
I decided I wouldn't be interested in it after looking at it 2 or 3 times.