The book makes sense as a history of penal reform, and Foucault is right to point out that changes often served to better punish prisoners rather than reform them as the rhetoric of the day demanded. But pointing out public hypocrisy is hardly a sophisticated observation for a historian to make. And it is a weak base from which to launch the sweeping attack Foucault attempts on the idea that morality matters, or to insist that man's "soul" is nothing more than his position in power networks or that "power" itself is "knowledge" (not vice versa).
In order to support these points, Foucault is dishonest on so many basic historical points that it calls the honesty of his actual historical research into question. I have a ton of underlined passages in my copy of D&P which I made with increasing frequency as his claims started to contradict each other. In the interest of just picking one, look at Foucault's insistence that democratization had absolutely nothing to do with efforts to remove the death penalty. Really? And Foucault "proves" his opinion not by citing a single document but rather by simply stating his opinion that "executions did not, in fact, frighten the people."
Well which is it? Are we truly to believe that the horrific demonstrations of torture which Foucault spends entire chapters chronicling (salaciously, like pornography) did not frighten anyone? Why if so does he describe them in such detailed fashion? And doesn't his own claim about the ineffectiveness of the punishment now contradict his earlier statements about the nature of knowledge, to say nothing of his earlier chapters which actually documented how people were, in practice, quite terrorized.
A lot of the dishonesty slides by in the sheer minutiae of death. And it all gives the impression that Foucault must be right, if only because he is able to a picture of humanity that would repulse most honest people and do so unflinchingly (the virtue of facing unpleasant truths!). But at a certain point, the discerning reader recognizes his sophistry for what it is and stops trying to make sense of it all. I'd encourage anyone on HN to go directly to Plato if they want real philosophy, or at least jump back to Nietzsche (Schopenhauer as Educator) if they want an entertaining and tongue-in-cheek iconoclast whose ideas merit real thought.