It’s pretty useless in the sense that they are trying to seek rent on the basic concept of location; per their ToS you are not allowed to cache lookups (so every lookup must go through them), not allowed to reverse-engineer the mapping (not sure if that would hold up in court; software patents aren’t a thing in the EU as far as I know) and their business model is to drum up PR and get mass adoption before starting to charge for lookups. It also has some technical shortcomings like not accounting for continental drift, so in certain locations your W3W will change over time which is not ideal for something that wants to become a better address system.
Their PR attempts are all terrible, they rely on sob stories where someone is injured/in an emergency, has access to a phone, GPS signal, the app and a network connection but somehow needs W3W to figure out where they are. If you have GPS and network why not just use GPS coordinates, or even just click a link provided by the emergency department that would automatically send them your position (if it hasn’t done so already - I believe phones have an out of band way to automatically send their GPS location when making an emergency call, at least in the US).
Here’s a blog post explaining this terrible idea in more detail.