There are
other companies and universities that are working on other combinations.... basically we will just pick whatever has the best cost/performance ratio... assuming that the product can charge quickly. The thinking is we will have a matrix of these.. some will be charging while other are discharging. A computer will cycle through which caps are charging and which are ready to discharged.
So once these caps are charged they will keep their charge as long as we are able to recoup energy lost from accelerating, drag, and accessories.
Another major technology will be special struts that are able to convert kinetic energy back to electrical and that are able to damper the vibrations in the suspension.
Yet another idea is to use a special paint that is photovoltaic to help aid in energy efficiency. This is an exciting new technology that we should all see in the upcoming years in small devices like cell phones and mp3 players.
so mileage will be dependent on the size of the vehicle... the weight of the cells... and how much you want to trade off weight for capacity. I am sure we will find a sweet spot as far as power vs weight vs cost.
Also we should start seeing super caps replacing li-ion batteries in nearly every field especially light devices like cell phones.