I've got a Brompton folding bike and it's really a great way to move through a city.
It's not as compact as your device, but it gets quite small when folded for storage, and there's a configuration where you fold it down except for the handlebars and can push it around like a little cart.
I can second that a Brompton is an awesome city bike. I had one in Boston, and besides the aforementioned utility, it got compliments from people from every possible demographic. They put a smile on damn near everyone's faces.
Get a good pair of padded gloves though. The small, high pressure tires are a punishing ride over shitty pavement. I.e., the kind that's found everywhere in Boston.
It's not as compact as your device, but it gets quite small when folded for storage, and there's a configuration where you fold it down except for the handlebars and can push it around like a little cart.