That said, it is not impossible to recognise that he has certain virtues as a philosopher, which I would rate pretty highly, though ultimately they are limited. His charm, which is quit specific, is a driving hostility to received opinion and fossilised thinking, which pulls him forward in an entertainingly energetic way. Like Heidegger, he has an instinct for the way that conceptual development takes one away from being. In the end, though, like most 'deconstruction' his nihilism only destroys, making a fetish out of stupidity.
Charles is right on Foucault, too. There are some interesting things there, but he is definitely motivated, and said as much, I think, by a desire to break with Marxism, and we ought at least to honour his intentions in that regard. It is pointed that though his reputation is secure among cultural theorists, and indeed is still growing in other disciplines, like law and International Relations, to name but two, his specific scholarly contributions to cultural history are ripe for demolition.
In the Times Literary Supplement, Andrew Scull demolished Foucault's History of Madness, recently (http://tls.timesonline.co.uk/article/0,,25347-2626687,00.html). Also his History of Sexuality is in my (admittedly worthless) opinion too mulishly partisan to be taken seriously except as polemic. I mean that what it presents itself as, a comprehensive history of sexuality, is precisely what it is not. Rather it is a series of interesting interventions into a debate, notably on Freud's repressive hypothesis (which is pretty interesting, and productive, but sets up something of a straw man when one considers Freud's care in constructing the idea); and on Greek Homosexuality - though I believe this is largely derivative of Kenneth Dover's book on the same. Again, Foucault is very imaginative and forces us to look at things anew, but he really is not trustworthy as an historical source (which is quite a failing).
For my own part, I think he is at his best when seen as an intervention into contemporary debates rather than as an historian. And for my money, it is his single minded libertarianism (in the social rather than the economic sense) that really recommends him. There is an interesting controversy about Foucault and AIDS (aired I think on this list a while ago) which sheds light. He is reported, by biographer James Miller, to have rubbished the (then-named) Gay-Related Immune Deficiency Syndrome as plainly an establishment plot to demonise sexual adventurism. On this point, of course, he was tragically mistaken, and these things are remembered usually to ridicule Foucault. For myself, and I admit this is a stretch, I think if he was wrong on the medical science, he was right on the broader point, that the AIDS health emergency was used to demonise sexual libertarianism in a destructive way. Confusingly, lots of Foucault's supporters, like Simon Watney, were wholly complicit in fanning the flames of the moral panic over AIDS, talking up fears of a heterosexual epidemic to wring funds for their Safe Sex campaigns. Foucault, who Miller reports as saying, 'besides, what could be more marvellous than to die for the love of a beautiful boy' (paraphrase) would surely have been repulsed by the Safe Sex campaigners.