Zamora's point is important, but it is not the final point, and as with Nietzsche, the complete denunciation of Foucault is a bit unfair. Chomsky put it best: Foucault's general argument was always
that there has been "a great change from harsh mechanisms of repression to more subtle mechanisms by which people come to do" what the powerful want, even enthusiastically. That's true enough, in fact, utter truism...Some of Foucault's particular examples (say, about 18th century techniques of punishment) look interesting, and worth investigating as to their accuracy.
This is the Foucault I'm most familiar with: the Foucault who argued that power is subtle, persistent, and complex, that it can only be slowed down, and that it inevitably co-opts the very institutions built to contain it. Which, as Chomsky notes, is a truism, but the historical examples that Foucault explored (sexuality, psychology) demonstrated this truism's scope and implication in ways that had not elsewhere been so thoroughly explored. Today we are immersed in an intellectual climate that is constantly hunting out ever-more-obscure microaggressions and ever-more-elaborate forms of privilege; we take this project for granted, and can easily forget that Foucault was in many ways its pioneer.
That project, in its ever-expansive complication, often seems to be in conflict with the Marxist effort to understand the complexity of domination through the basic framework of class struggle. And this, of course, is precisely where Foucault is at odds with the left: his failure to relate the multiplicity of power back to its basis in the material economy.
But that does not mean that we have nothing to learn from Foucault, or from today's intersectionalists and deconstructionists. They remind us that class struggle is rarely simple or transparent. The domination of the bourgeoisie is sophisticated, minute, opaque, and constantly evolving. When we understand this, we aren't caught off-guard or distracted from the class war by every new tentacle of oppression that bursts from the sea; our focus remains on the monstrous bourgeois beast beneath the surface.
Consider, for example, this recent article by Corey Robin, which exposes yet another form of bourgeois domination: employers are forcing employees to support particular political campaigns. This, he notes, has been largely overlooked by critics: "By focusing so much attention to [Citizens United], critics misstate the actual problem of corporate power and political influence."
This is the sort of problem you run into when you have an overly-rigid and narrow conception of power. This is a particular handicap for liberals: they think of our liberal democracy as essentially benign, so they maintain a reactive political disposition that only notices various "corruptions" given a sufficiently eloquent article or a sufficiently aggressive agitprop campaign. But this can also happen with the leftist who misunderstands class struggle as a simple matter of low wages and secret bribes, and misses the less obvious ways that it's manifest in our society. The underlying problem, Robin continues, is capitalism:
Workers are dependent on employers for their well-being. This makes them vulnerable to their bosses' demands, about a great many matters, including politics. The ballot and the buck are fused. Not because of campaign donations but because of the unequal relationship between capital and labor.
It's easy to see how that unequal relationship will always, constantly find new ways to express itself in our politics, circumventing every attempt we make to regulate it. If you understand the basic class dynamic at work here, then you'll find Robin's article interesting, but not particularly surprising: it's just the latest way that the bourgeoisie has found to leverage its control over the means of production. The underlying dynamic is the exact same as the one Marx noticed at work over a century ago.
Ironically, there's a strand of conservative thought that knows this as well: the one that sees attempts to govern capitalism as futile, since it's so innovative and persistent. Laws and regulations, they argue, have perverse and unexpected consequences that lawmakers can never anticipate; and in the end they'll only hurt law-abiding citizens since criminals will inevitably find ways around them. These are precisely the sort of insights that Foucault found so seductive: the vision of power that's more cunning than we are, that can draw us along an endless intellectual chase after its ever-more arcane and counterintuitive machinations.
The left doesn't need to get drawn into that chase. We can watch Foucault and his successors run after power, we can take their discoveries as reminders of just how pervasive and evasive power really is -- and then we can remind ourselves that our task isn't just about regulating or containing capitalism. We're here to destroy it.