I refer to "The Pianist's Guide" frequently, using it as a resource for discovering "new" repertoire for my students, organizing the literature they study into levels that progress smoothly from easier to more difficult literature, and exposing them to some wonderful composers that are perhaps less famous than great masters such as Bach, Mozart, and Beethoven with whom they are most familiar. I also use this reference as a way to find, for example, all of the Level 2 pieces from the Classical period (I keep lists of these searches, which we began in graduate school, in a binder arranged according to level), especially when I am teaching multiple students who are studying music of the same level of difficulty and who need to play a variety of pieces for a specific recital or festival.
Using the levels from "The Pianist's Guide" as a standard gives me the luxury of maintaining one organized way of thinking about the many levels of elementary through early-advanced literature, especially when I am teaching from a variety of different collections and using many different series that each assign their own levels to their specific teaching materials. This organized thought process helps to ensure that I assign literature to my students that progresses smoothly from level to level and that I provide pieces for them that gradually move from elementary literature into more advanced literature in an organized and sequential fashion.
There are several collections of standard literature Jane Magrath has compiled and edited that follow the leveling system of "The Pianist's Guide". Stay tuned for more information on these wonderful collections!