The Six Ideas That Shaped Physics series is one of those physics textbooks that tries very hard to change how students think about physics. And maybe it represents the start of a new trend in physics education: introducing the great conservation laws of mechanics instead of Newton's laws in an attempt to introduce students to the heart of physics. But on the other hand, the old guard of physics texts, Kleppner's An Introduction to Mechanics and The Feynman Lectures on Physics, are too timeless to put aside. When I picked up Unit C the summer after taking calculus based physics as a high school senior, I was impressed by the approach to mechanics I had never seen before. But after a year of honors introductory physics using the Six Ideas Series in college, my appreciation for Moore's unique textbooks cooled. That is, his books were useful only once.
Unit C: Conservation Laws Constrain Interactions covers the bare minimum for any student's physics education: vectors, energy, and linear and angular momentum. Notably, Moore refuses to cover forces in any great extent in his first text of the series, which for a person (like myself) with a strong introductory physics background, should be highly novel and interesting. However, I can only imagine the complexities this approach presents to a student trying to grasp physics for the first time, and my own experience taught me the book was useless for one who has already gone through it once!
The major fault with this textbook (and with the whole series), is that the reader is not left with a "deep" understanding of physics. Some basic situations and principles are shown, such as a pair of billiard balls colliding on a table, but the sort of physical intuition needed to solve complicated problems is left out amid the conversational prose and nondescript end of chapter problems. From Unit C, you will certainly learn what the conservation of momentum looks like mathematically, you will understand what a one-dimensional potential energy well is, but the book will likely not give you the enlightenment necessary for solving challenging problems. You will need Kleppner and Kolenkow for that.
The Six Ideas series textbooks, and Unit C in particular, are good reads for the individual who has a reasonable physics background and wants to solidify it through a summer of light reading. In particular, the two page pre-chapter overviews and two minute post-chapter problems make the book valuable to the self-studier; it would be wonderful if other textbooks used a similar arrangement. But beyond the novel perspective, the broad approach to mechanics ranging from the conservation of linear momentum to the brief overview of thermal energy contained in this text, and the bells and whistles Moore's student-centered approach provides, the book is just one more of many introductory textbooks out there.
So if you are studying physics over the summer, go ahead and purchase the series. Otherwise, the classics are just too good to replace.