The Bastien piano series are a very useful tool for piano students. Although the Bastien Method is not my go-to method book for my personal studio (my preferred method at the moment is Faber), I use this series as supplementary material and have used it as a main method book in the past when I saw that students had a good ear for music. In addition a music school I worked for in Italy used these books regularly, so it is a reputable method series for those studying classical music. I do not currently own the upper levels of Bastien but am familiar with some early level books, so my review is based only on those.
Bastien Method Books are well-suited to ear players. Students will enjoy learning about intervals and chords in the primer book. Soon after they will learn about major and minor pentascales. Learning scales and chords as an ear player is important because with an understanding of chord progressions (both tactically and theoretically), students can better compose their own music. Tactically, students will learn a simple formula for playing basic chords in a variety of keys. Theoretically, students will understand, for example, why a left hand C chord is needed when the right hand is playing C, E and G in a melody.
The Bastien Method Books are well suited to a variety of students, particularly as supplementary material. Although I wouldn’t recommend it much for small children due to it’s quick pace, school aged children and even adults can benefit from the Bastien books. One part of the Bastien series which I’ve used often are exercises in the Theory 1 book which teach students how to play a right hand notated melody while playing a chordal accompaniment (following the I & V7 symbols). These exercises are a great introduction to lead sheets! As far as the lesson books go, pieces included are pleasing to the ear, including both major and minor sounds. Students will also enjoy the early sense of satisfaction when they play familiar songs using both hands even from the primer level.
As a teacher I enjoy using the Bastien series as a springboard to teach various theory concepts mentioned above. Rhythm is introduced very quickly in this series so if you do use it be sure students truly learn the rhythms rather than copy what they hear you play or what they know a popular tune sounds like. All in all I think Bastien books are a great aid to piano teaching and study!