I must say, following your previous mention of Ingress, the basic idea has caught my imagination more than any other in recent years.
This might, in time, be a genre that will gain mainstream popularity in the same way that the Wii console did. Partly it is the health aspect of getting out and having a reason to walk around your own area but, also, it could be a really good match for travel – imagine flying into Warsaw for the weekend to play an espionage-themed version. If cleverly laid out, it could be a terrific way to experience a city for the first time. Edinburgh’s old town, for instance, would be an epic backdrop.