I used to be obsessed with finding out what kind of real thoughts, really important ideas postmodernism, poststructuralism etc. had to offer. I read a lot of "French theory" and secondary literature - not enough to have a really thorough grounding, but enough, I think, to be able to form my own judgements. Even though I can't remember most of it anymore, I don't think it was a waste of time. If you're not trying, maybe not daily but constantly, to test your own worldview for emergent dogmatism, you're doing it wrong. It's always worth it to kick something outside the borders of your worldview and see if it kicks back, although not every something is an equally worthy candidate.
Having read a bunch of original monographs and articles, a a stack of scathing critiques and even more scathing countercritiques, having went through two graduate seminars on Derrida and postmodernism (not for credit - I was studying math at the time), luckily with a professor who welcomed criticism, here's what my very personal opinion comes down to:
1. Most of it is bunk. There's just no escaping it. However, that's not necessarily a strong point against it, because see Sturgeon's Law.
2. Occassionally, some of it contains real insight, thoughts of depth and value; unfortunately, you have to work really hard to get to them, as they're shrouded by murkiness and vagueness to a degree that's quite astonishing. The thing is, postmodernists will tell you (and, in so many words, so will the primary texts) that this vagueness and murkiness is necessary; that there's no way to approach the deep thoughts except by deconstructing language and thought themselves. Well, I ain't buying that (I've tried very hard to buy that, it just won't get bought). In all cases where I actually reached something I thought was valuable, all the junk on the way only hindered, never helped - the horrible jargon, the bureaucratic sentences, the nonsensical code-words, the weak and unstable pulse of the argument. I remain convinced that none of this is necessary, and it's there simply because the culture encourages bad writing, in which it's easier to hide pure bunk (see 1.).
I offer Derrida as an example of a postmodern (well, poststructuralist) author who has Something To Say. In particular, after some wrangling and exasperation, I grew to appreciate his Dissemination, especially the first part, Plato's Pharmacy, which offers some brilliant observations on Phaedrus and the nature of writing (again, if you can plough your way through to them, etc.). I recommend this. I've never read his more famous Of Grammatology, and not sure I will now.
3. The quality of writing varies wildly from one author to another, even if we restrict to the most famous ones. Lacan is bullshit, pure and simple. Don't waste your time on him. Foucault seems to have been a shoddy scholar whose theories fall apart when you look at the evidence closer. Baudrillard I've come to regard as a sort of a gifted stand-up artist with an ear for impressive aphorism.
With all the occassional good stuff I found while reading postmodernist literature and arguing with friends about it, I couldn't possibly "follow" it in any meaningful way because of these (again, my personal opinion, etc.) flaws:
a) culture that accepts and encourages texts of very poor quality. This is more damning than 1. above; this makes sure the shit rises to the top. Most of writing is so obscure that it's not possible to refute it: there's nothing solid to get hold of to refute. Behind this obscurity is a huge amount of parroting, bad thinking and simply nonsense, and nothing in the intellectual postmodenist life encourages you to try to separate the wheat from the chaff; in fact, you're the enemy if you think wheat and chaff can possibly be separated, or even exist in any objective way. Think of the Bogdanov affair others recalled in comments here. In Sokal, outsiders (a physicist, other scientists, journalists) criticized the "postmodernists", who counter-attacked in the "culture wars". In Bogdanov, physicists and mathematicians themselves attacked the apparently nonsensical work, and the debate was between scientists. There's a huge difference here.
b) the double standards that pervade all texts and thought. Postmodern authors almost never scrutinize their own assumptions as critically as they scrutinize, say, the assumptions of modernists. It gets ridiculous after a while, because you find it hard to believe that someone can, say, claim that all ideologies are equally suspect and we must denounce the very possibility of knowledge itself if we are to get to the bottom of it all; and then in the next paragraph prostrate themselves dogmatically before some other thinker and quote their claims with all the reverence of Gospel truth, obviously bearing universally validity. You keep thinking, wait, it must be a joke, but the punchline never comes. Another way in which this annoyed me was how inevitably almost all those guys were ideologically extreme leftists, usually Marxists. It's not even the ideology itself that irked me the most, it's how unimaginable it was to see anything else, how dogmatically it pervaded everything, and never got a fraction of critical attention that was lavishly spent on, say, the idea of objective truth, or the simulacra in modern capitalist society. These are just two most obvious examples, but really, the amount of double standard going on, the lack of reflection at one's own thoughts is very irritating. At the end, I couldn't accept it even as a worthy way of exploring the world that's simply alien to me. I couldn't escape the conclusion that almost all of it was junk, sometimes very attractive intellectually - and therefore dangerous - junk; with some surprising exceptions, like some of Derrida I talked of above, which don't, after all, change the overall picture.