Scott McCloud does a fantastic job explaining the history, potential, and inner workings of comics as a medium. I was especially impressed with his concise descriptions of visual theory and its particular applications to comics. Occasionally I felt that McCloud's treatment of a topic could have been more fleshed-out (the chapter on color, for example, or his concluding idea of comics as a particularly good form of communication) or that he made some unnecessary generalizations (his definition of art was a bit trite and even misleading). On the whole, though, McCloud's ideas are sophisticated and he is able to communicate them with surprising eloquence to both the art historian and the general public. In fact, though I am an art historian, I learned a good deal from this book.
McCloud's decision to use the comic format to present his ideas is ingenious, and I doubt that prose alone would have been able to deliver his messages with such clarity. The one drawback to the format is that I fear it will only appeal to those who already value comics, and that as a result those who most need to hear what McCloud has to say never will!