It is a great struggle to find a way to evaluate presidencies that is not colored entirely by our own political inclinations. As a lefty, I tend to see liberal presidents with liberal achievements as the good presidents and conservative presidents as the bad ones; however, Greenstein is trying to develop a framework of judging presidencies based on factors that are not connected to ideology.
The framework he develops makes a good deal of sense. It measures six dimensions of presidential leadership, including vision, cognitive ability, emotional intelligence, and political skill.
It's the execution that bothers me more than the device. The chapters are too short to really dig into why any particular president is considered to be successful or not, and the application of each category feels arbitrary. In addition, by seeing a presidency only through these six lenses, most of the responsibility for the presidency falls only on the shoulders of the individual president, rather than acknowledging the importance of external events and the zeitgeist of the times that can be important factors.
Greenstein's framework can be a valuable tool, but I think it is up someone else to take the ideas he has developed and further improve them. The project of finding objective ways to judge presidencies is a very difficult problem to solve, and this book is a step toward that goal, but we still have some ways to go.