Segways aren’t particularly useful but their balancing system is pretty nifty: http://science.howstuffworks.com/engineering/civil/ginger1.h...

You don't need to worry about parking your segway outside the place your going to if you enter the building with it. So, opposed to using a bicycle or scooter, someone stealing it while you're away is not an issue.

It's upright, so you can be seen crossing the street by motorists, which gives it an advantage over a motorized wheel chair.

It saves energy compared to driving a car and it's use would greatly reduce traffic where cars are normally used.