Been using is for several months. Definitely VERY overpriced (I’d say $3-4/mo for a search engine would be fine, not $10), but the results are great, and I love the quick answer feature. It quickly summarizes info from top results, helped me a lot in college, where sometimes your brain is melting and you want the answer NOW.
The problem I see with federated wikis is potential creation of echo chambers. Current Wikipedia is often a political tug-of-war between different ideological crowds. For instance, on Russian Wikipedia, Russian Civil War article is an infamous point of struggle between communist and monarchist sympathizers, who often have to settle at something resembling a compromise.
If both sides had their own wikis, each would have very biased interpretation of events. A person who identifies as either communist or monarchist would visit only the corresponding wiki, only seeing narrative that fits into their current world view, never being exposed to opposing opinions.