My wife and I bought a pair of Segways three years ago. They are intensely fun and very useful for exploring new areas when sightseeing. You can cover a small town in a couple hours and, unlike when using a car, you are able to really see everything around you and stop as often as you like without causing a traffic jam. They are much more maneuverable than a bicycle, allowing you to stop anywhere without fear of falling over. You are also a foot taller than you've ever been, able to see over fences, etc., instead of ending up being shorter, as you are in a car or bike.
Segways are typically permitted on sidewalks as well as streets, and we have had ours in five-star hotels, as well. The tires are made of silicon, not rubber, expressly so they won't leave black marks on marble floors. It is so much fun to cruise up to a fancy hotel, your roll-along suitcase trailing from one hand, up the handicap ramp, through the handicap door, across the lobby to the registration desk. You check in, still standing on your Segway, then cruise right onto the elevator, wheel around, press your floor, and the next thing you know you're cruising down the hallway and easing into your room.
The downside is service: We've had two battery failures, both under warrantee, as well as a gyro failure, also under warrantee. The problem is you have to slog the Segway to an authorized dealer. Batteries the dealer can replace, but anything else requires a month-to-six-week trip back to the factory. Once you've gotten used to using your Segway all the time, this is intolerable. Unfortunately, we're hooked, so no matter what it takes, we'll want our Segway back.
If you don't know whether you want one, take a Segway tour of a city near you. They'll teach you how to use it, then take you on a trip around town. Much fun. We did one in DC, then one in Paris. Then, we bought.