When I've seen people unicycle (I'm a 2 wheel man), they go faster than a power walk but slower than a run. Plus, they're smaller and more maneuverable than a bike so they're much easier to take on sidewalks and around pedestrians. Pretty similar to a Segway.

Pedals drive the wheel directly (no gears), so to slow down or stop you just pedal slower and then hop off.

Wikipedia (http://en.wikipedia.org/wiki/Unicycle) is pretty fascinating, including these notable unicyclists: Adam Corolla, Donald Rumsfield, and Claude Shannon!

I can sustain about 12 mph, so it's about 2/3 as fast as biking. Some harder-core unicyclists can get over 20 mph with extra-short pedal cranks or gearing.

Almost all pedals are 1-1, which is why you want a big wheel. You brake by resisting rotation while you tilt back.