Has anyone successfully used this for anything significant? As I post this there's only a couple of people posting about how either they tried but it didn't work out well enough, or they just played with it in school without any code.
It strikes me as having the same problems as RDF storage; precisely because the model is so general, there's nothing for optimizations to grab on to and optimize on. Your database has to stand ready to do anything equally. The core of database optimization is, when you really get down to it, working out how to tell your database which things you really want and what you will never care about, as in "This table has twenty columns but I will only query on name and phone number". In practice, limitations aren't always just there because of a lack of imagination or insufficient computer science education (though that does happen), sometimes they're there because you can't get any performance without them.