There are a few electric scooters and if you look at alternative fuel web sights much, you can see afew conversions of mopeds and a lot of parts from the defunct Electric Warrior. The problems with electric scooters and such can also be the effect of cold on the batteries. They will not hold as much of a charge in the cold. Depending on the efficiency of the charging system, you might be able to save a lot on fuel until you have to replace the batteries (as previousely noted).
The bennifit is not so much in fuel efficiency but in the eco, save the earth type of stuff. The idea is that there is no emisions from electric power. Electric motors also have a greater efficiency curve as compaired to gasoline motors. The motor will be a little lighter but the batteries will certainly make up for the differance. With an electric motor, you also have a slightly differant rating for power. People try to create a hp rating for electric motors but it really doesn't work quite the same. A 2hp rated electric motor may be able to put out as much as 3hp for a while. This can potentially cause excess heat in the controller but most controllers are used in such away that they are everated for the power that will go through them. ie 120 amp controller when an 80amp is the rate needed. The technology for the controllers are also the driving force in the advancement of electric motors for propultion. Instead of using a rheostat, they generally use a type of mosfit controller that gives varying pulses of electricity to the motor. Before the motor completes the cycle of the rotation, the controller has pulsed the power a certain number of times. (Its a little more complicated than that actually but no real need to go into the controller technology). Instead of having a throttle, you have a potentiometer (like the volume control on a radio).
Anyway, the batteries have and will be for a while the downfall of electric scooters and automobiles until the batteries get even better and cheaper or until fuel cell technology gets better. You can actually use the hydrogen in a fuel cell to create electricity to drive an electric motor. With this type of conversion, the only emission is water!!
If you consider buying the scooter, take care of the batteries and keep up with the number of charges used for fuel efficiency. I am sure that you will find that the old internal combustion engine is still the way to save on money. One day though, we will be talking about electrical parts and not carbs and exhausts and stuff.