I read this in college and didn't care for it. I read it again a decade later when I had gone back to college and really liked. Having learned much more about the American Civil War in the intervening decade probably helped.
McPherson's thesis is that, because they lived in a politically-active pre-cynicism era and were mostly volunteers (rather than regulars or conscripts), the soldiers on both sides of the Civil War were highly motivated by patriotism, duty/honor, and political ideology throughout the war.
I put this book in my top five most important books for understanding the war.