I guess the main reason to buy this would either be cost savings or environmental-friendliness.
The numbers for this on Gasoline Gallon Equivalent mileage would be ~800 miles to the gallon.
It uses an 8KWh battery to get 200 miles. A gallon of gas is ~32KWh.
The Miles Per Gasoline Gallon equiv. would be 2,666($4 per gallon/.15 per KWh). So, price-wise, it is equal to a 2,666mpg pure gasoline car with those prices.