Marxian critical theory is a powerful tool for analysis.  As much as Marx's normative politics are on crack (IMHO, of course) owing to the whole homo economicus stuff ("man's essence is to labor and create shit and then own it"), the critical stuff that stems from him and his students is fabulous.

I'd suggest that proof of the power of the tool is its reproduction all over the political map (notably the righty Christian critical method of the "worldview," which is just dumbed down althusserian critique of ideology)