I'm in the UK. I have all except a hospital, movie theatre, university, and sports arena within a 15 min walk of my house. I live in a suburb in a large city (not London).
I can get to a hospital, movie theatre, and a university within 20 mins on a bus. I can get to 3 sports areas in about 30-40 mins on a bus in different directions.
My area is not a 15-min city, but is moving that way. My own street has been split up and blocked to through traffic (which I love), and there is proposal for a pollution tax to dissuade polluting traffic in the city.
But at the moment I cannot realistically commute to work as the public transport capacity is just too low to make it comfortable and safe (I work early and late hours).
I think the concept is great but I suspect they need most of the elements to be in place before they can achieve critical mass and change behaviour. Certainly for me I'm still doing a lot of driving.