From a end-user's perspective, it was perfect! It was gamified just enough to be engaging but not so much that it would become annoying or a grind. It was tied to the real-world via restaurants and other places offering special discounts when you "checked-in" or became the mayor of a place. It had people contributing essentially-real-time updates to metadata of all places (opening hours, location, description, phone numbers, images, etc) - way before Google Maps user contributions were really a thing. It had a social network aspect to it that actually worked etc etc..
It pains me that Foursquare, the company, failed to figure out how to actually monetize any of this and keep the thing going that made them popular.. so they grasped as straws and did the whole app split thing (WTH, really) and the community just faded out as far as I can tell.
Dennis is trying something new now, which is true(to him) and to the original vision of Marauder's map and FSQ. He has been on the dogeball train since a long time and hats off to his commitment.