Would love to hear people's experience with it, especially for more complex things. Seen it used a few times to add consistent metadata to pages (through forms), but never much built on top of such basics.
Where extensions like SMW really shine is for wikis which discuss highly structured content, like content in video games. Another extension which I've seen used in this space is Cargo [1].
We never bothered doing anything like concepts, detailed types, etc...we only used it for basically adding tags to pages that you could query as part of a search. It worked okay with that. We had some issues where we wished a property could be an enumeration or nullable, which I don't think ever happened in one of their many releases.
Adding the metadata represented a bit of a challenge. We eventually had it generated from templates that would usually get filled out anyway.
It was, in a nutshell, not great: there's a fundamental contradiction between wikis, which are very loosely structured and edited by humans who do all sorts of wacky unpredictable shit, and hierarchies, which need to be carefully pruned and maintained.
So this use case didn't work, and I think it's telling that (AFAIK) not a single Wikimedia project has adopted it.
SemanticMediaWiki has some questionable scalability when it comes to large sites. Wikimedia was never going to adopt it for technical reasons.
The advantage of SMW imho is still that it brings the semantic / structured data features right into the MediaWiki itself and that there's a big ecosystem of related extensions around it (like Page Forms). So far I don't know of an open-source information management system which comes close in capability to this. Setting this all up and maintaining it is difficult, though - I even had to start developing custom tools to assist me with that once it got more complex.
However if we're talking about Wikipedia, any scaling concerns at all basically make it a non-starter.
https://wiki.guildwars2.com/wiki/Guild_Wars_2_Wiki:Reporting...