Hands up anyone who uses RDF (when they could choose to do otherwise)? Anyone? Bueller?
That said, RDF has historically had three main problems, IMHO. The first is that its proponents have, historically speaking, done a crappy job of explaining what it actually is, and a worse job of demonstrating what it can do.
The second problem is that it's not really the most human-readable format around, especially in its XML serialization format. This is largely because it wasn't designed for human-readability, but in the age of the Web (where human-readable formats have shown to have major advantages over non-human-readable ones), this is a liability. They've added some more readable serialization formats over the years, but a lot of the documentation and tutorials assume that you'll be working in XML.
When I first tried to learn about RDF, years ago, all I found were tutorials full of really gnarly-looking XML with minimal explanation of what was going on. I spent so much time getting bogged down by the syntax that I missed the point entirely.
The third, and bigger, problem with RDF, is that it's a little hard to "grok" if you don't have a good grounding in predicate logic, description logics, and some of its other theoretical underpinnings. Actually, wait, maybe I said that wrong- I should say instead that most of the old-school RDF people do have solid theoretical backgrounds in description and predicate logic, and have a hard time talking about RDF and other semweb technologies with people who people who don't have that background. So they use lots of jargon that, while useful, isn't very helpful to somebody just trying to get their feet wet.
It's a little bit dated at this point, but Shelley Powers' "Practical RDF" was also helpful- but, again, I got a lot more out of it once I'd read Antoniou and had internalized the whole RDF thing at a "30,000 foot" level.
BTW, I just Googled for the Antoniou book to make sure I had remembered its authors correctly, and it turns out that one of the first results is a PDF version of the entire text. I don't know what it's doing up online, but grab it while it's hot. If it's helpful, Powell's has a couple of used copies:
http://bergie.iki.fi/blog/using_rdfa_to_make_a_web_page_edit...
The content repository we use also provides RDF storage and querying:
http://www.midgard-project.org/updates/midgardcr_10-12-hrung...
I've used it for past projects (NDA, can't really talk much about it here) in the knowledge representation and inference domain.
I'm at work at the $DAYJOB right now, so I can't really do anything with this right now. But for what it's worth, one of the areas I'm working with is something called SKOS. http://www.w3.org/2004/02/skos/