Compare it to a moped: much slower, much shorter range, can't carry a passenger, can't carry cargo, can't travel on roadways, much more expensive.

Compare it to a bicycle: slower, much shorter range, can't carry cargo, can't travel on roadways, very much more expensive, can't be used for exercise, going up hills is easier.

Compare it to a car: enormously slower, vastly shorter range, can't carry cargo, can't travel on roadways, exposed to the elements, potentially comparable cost.

Compare it to walking: saves some time and effort especially on hills, less convenient, borderline rude to use near other pedestrians, avoids extremely low impact exercise, still exposed to the elements, very high cost.

And there you have it. For the vast majority of situations using a Segway is actually a huge downgrade from an equivalent expenditure. For the cost of a Segway you could buy a used car, or a moped, or an extremely nice bicycle. Meanwhile, if you have a commute that you would be willing to use a Segway for it would generally be absolutely superior to simply walk or bike and get some exercise in the process.

I'm a perfect candidate for a Segway -- I live in a small city, with most of my key destinations 30-60 minutes by foot from my house. The problem is, Segways were like $6k each, so I'm going to have to drop $12k for my wife and I to go to work, still need a car, and not be able to legally operate on the sidewalk.

...Or, I could drop $800 on bikes.