I've looked into Semantic Web Technologies for a year now and trying to come to a personal conclusion at the moment. This is my current state, through some of this may be premature:

PRO:

* I can see that the semantic annotation part of it is spreading. Schema.org / JSON-LD might be the first pragmatic solution that I can imagine actually getting more widespread acceptance. Especially if currently existing Frameworks / CMS add support by default.

* Semantic Annotations are helping big companies like Google to make their products smarter and this is happening right now.

* Semantic Web tries to solve some real problems, not just "academic" problems. Information and Knowledge is indeed rather unconnected which reduces its value tremendously. Right now APIs grow to make this mor accessible, but there are many problems unsolved.

* SemanticWeb has some truly interesting ideas and concepts, that I've grown to like. Of course nearly every one of them could work without buying the whole Semantic Web. But still, I think some very interesting ideas come out of that community.

CON:

* It takes a lot of time to understand the Semantic Web correctly and learning about the technologies behind gets soon very mixed up with a lot of complicated and rather uncommon concepts, like Ontologies.

* The tools (even Triplestores) feel awkward and years behind to what I'm used to as a web developer. There are a LOT of tools, but most seem to be abandoned research project which I wouldn't dare to use in production.

* It gets especially complicated when entering the territory of the Open World Assumption (OWA) and the implications that has on reasoning and logic. Say you want hard (real-time) validation because data is entered through a form on a website. Asking some people from the Semantic Web Domain, the answers varied from "I don't know how to do this" up to "Its complicated, but there is some research... , additional ontology for that...". I'm kind of shocked since this is one of the most trivial and common thing to do in the web. And I really don't want to add another complex layer onto an already complex system just to solve simple problems. Something's wrong with the architecture here.

* OWA might be interesting, but most applications / databases are closed world and it would make many things very complicated to try fit it into the Open World Logic. OWA is an interesting concept and makes sense if you are building a distibuted knowledge graph (Which is a huge task and only few have the ressources to do it), but most people will want to stay closed world just because its much more easy to handle. The Semantic Web seems to ignore reality here and prefers to be idealistic, imho.

* This sums up to me to one big problem: The Semantic Web Technolgies provide solutions to some complex problems, but also make some very easy things hard to do. As long as it doesn't provide some smart solutions (with a reasonable amount of learning / implementation time) to existing problems, I don't see that it will be adopted by the typical web developer.

* There are not enough pragmatic persons around in the community, that actually get nice things done that produce that "I wan't that, too!" effect.