This is an amazing series of books for any age beginners of Oral English as a Second Language (American English). I taught this series in China for 3 years, and am currently an M.Ed student. This series not only allows an inexperienced teacher to be able to jump into any class and feel confident teaching, but offers a strong order to the subject matter that is taught. The vocabulary and grammatical forms that are introduced are very useful in daily life.
If I were to offer criticism about this series is that these books are very sterile. Without supplemental activities, students will find the repetitive 'conversations' quite droll. Secondly, although it can be used at all ages, it's better to be used as a young adult/elementary book to be used as a SUPPLEMENT to other regular English classes. Without further instruction, many students feel that the material progresses much too rapidly. For example, Lesson 1 teaches the Alphabet and "What's your name?" while Lesson 3 is teaching Present Continuous Tense.