The challenge in studying semiotics or postmodernism is that, unlike studying literature, or history, or medicine, they are not first-order fields. Both of them are the study of the way in which we think, and as such have been very useful. Scholars like Pierre Bourdieu, Thomas Kuhn, Roland Barthes, Michel Foucault have really expanded our understanding of our own linguistic and cultural habits, the ways in which we both comprehend the world and limit our scope of action therein.
That said, there have been some truly awful intellectual and linguistic directions taken by postmodernism and semiotics: the former lends itself to a kind of nihilistic relativism which denies truth and meaning entirely; the latter to a kind of free-association in which things end up meaning rather the opposite of what everyone intuitively expects. There's junk science in every field, and these are relatively new fields; the ratio is still kind of high and both tend to attract "maverick" and "ooh, it's new and cool" types.
As I said before, the woo-use of semiotics draws on the way in which "signifier" and "signified" can be very different things: the way the flag stands for the nation, or "White House" stands for the presidency (or the nation). The wooists are taking that fairly straightforward process of unpacking meaning from language, and turning it into sympathetic magic. It's no different from their use of quantum mechanics, and you won't understand semiotics or postmodernism by reading Milgrom more than you'll understand Heisenberg.
One thing I didn't say is that postmodernists and semioticists have been responsible for some of the most opaque and bizarre prose in academic history, which is part of why they are so useful to voodoo peddlers. I remembered a piece I read back when post-modernism was just getting a foothold in US academia, and it was still called by its more linguistic term, "post-structuralism." It's a funny piece, still, for those of us who have to read this stuff:
TEN RULES FOR MAKING YOUR PROSE POSTSTRUCTURALIST:
Ruth and Kenny Mostern, Z Magazine, June 1991, p. 7.
1. Change all appearances of the verb "to be" to "can be represented as." Corrolary: Always refer to the word "is" as the copula.
2. Never "analyze"; always "deconstruct."
3. Never refer to "ideas" or "thoughts"; replace these concepts with "episteme," "habitus," or "ideological structure."
4. Actions are "always already overdetermined" by the categories in rule 3.
5. Feel free to add the following prefixes and suffixes to any word in your vocabulary: "post," "neo," "dis," "over," "quasi," "co," "de," "ism," "ize," "ify," "ness," "ology."
6. Use parentheses and dashes in the middle of words.
7. Every activity is "writing"; all things are "texts"; all people are "subject positions"; all collections of things are "structures"; all that is outside a structure is a "margin."
8. Conclude all discourse with several options and a question.
9. Call anything you don't understand "essentialist" and denounce it.
10. Refer to at least one of the following three French authors in everything you write: Foucault, Derrida, Lacan. Corollary: Appropriate all untranslated French words from your English versions of their texts.
Oh, that takes me back....
See also here and here and here.