My wife keeps asking for one of these. I think about 5 minutes of driving in New York City, and she'd change her mind pretty quick. (photo "borrowed" from http://www.monarchscooters.com/
Can't say about NYC, but in Seattle, those are vastly better than a bicycle and not really any worse than, say, a Harley or Gold Wing (except they don't climb steep hills as well, which isn't a big issue in NYC). Yes, they've invisible in traffic, but so are bicycles, and the messengers still get from A to B, pretty much all the time (it's the "pretty much" that makes a bicycle messenger's career short and exciting). So, for that matter, are full size motorcycles invisible in traffic -- I had people change lanes through my lane position a couple times a week for the entire year and a half I rode in Seattle, and my Hondas weren't any little flat-floor scooter with the engine under the luggage rack, even if they weren't a big old Harley or Kawasaki police bike.
In any case, for someone who grew up riding a bicycle in traffic, a motorcycle or scooter is no big change. For someone without that core experience, I wouldn't recommend driving any kind of two- or three-wheeler in a city -- you just don't have the level of paranoia necessary to stay healthy...