I've found that the concept of post-modernism tend to illicit strong emotions, either in full favor or in heated disagreement. I feel somewhat on a see-saw between the two disciplines as I've mostly studied economics and finance, which are considered the hardest of the soft sciences, and occasionally as the softest of the hard sciences. These fields rely heavily on both psychology and sociology, and calculus and statistics. Most people are lucky enough to not have a basic understanding about the concept, so I'll make an extremely simplified explanation.
Ever said, or have a friend say, "Everything is relative?" We probably all do, and on the bottom of that argument is the notion of post-modernism. There are no objective truths, which means that everything we know is subjective and open for interpretation. It often gets confused with the Einsteinian notion of relativity, though Einstein meant that reality is relative to the observer, not that reality is relative in and of itself. Einsteinian and postmodern relativity are important not to get confused.
From my experience, the hardest supporters of postmodernism come from the social sciences, especially continental philosophy and "soft" sciences, especially sociology and its many (many) related fields. They often try to push the idea onto the "hard" sciences (math, physics, chemistry, to a certain degree biology, etc), and state that these fields make claims that cannot be applied universally because there is no such thing as universally applicable rules. This usually pisses off the hard scientists to no end, which leads to events such as the Sokal affair.
In my opinion, the social (soft) scientists quite often needs to be careful of their criticism since most social scientist don't really have a firm grasp of the evidence requirements hard scientists face. It seems to me that most arguments usually end up in antI-scientism: Scientists have for the most part been white males, thus science, and the scientific model, is only applicable to white males. Yes, a lot of the criticism is this ludacris.
On the other hand, hard scientists usually aren't better. Quite often they want strict objectivity to be applied to the soft sciences. The problem with that approach is that soft sciences more often than hard sciences contain political, and thus policy, implications. And much like the rule set which describes micro and macro in physics doesn't correspond, the same applied for soft and hard sciences. Post-modernism can be both true and false at the same time. It may be an important philosophy within the social sciences and disregarded as bunk by the hard sciences, at the same time.
The major issue of applying strict objectivity to social sciences can be illustrated by our view of culture. Can you make a list of what makes an objectively superior culture? What measurements does an objectively superior culture achieve that an inferior culture doesn't? I can easily spot quite a few objective measurements in certain cultures vis-a-vis others, but I would hesitate to call one culture superior to another due to the political implications. Let's say I believe that my own culture is superior, which I could easily find academical support for, the policy implications is that I would have a responsibility towards other non-superior cultures. That leads to the idea behind "white man's burden", which I'd rather avoid. It's a good idea to be post-modernistic about it, and say that the Amazon stone age culture doesn't me to introduce them to medicine and iPhones, even though it would improve their lives on most objective measures.
As a conclusion I would state that those who view postmodernism as an all-encompassing theory probably need to venture outside their academies a bit more. And those who dismiss it as a useless piece of so-so may want to attempt to apply it into real world policy making. In any event, i believe different rules govern different magistra, at least until a more philosophically coherent mindset develops.