Lead acid is traditionally very forgiving with regards to temperature. How does the electrical performance of your battery vary with temperature, especially at extremely cold (-40C) and extremely hot (80C) conditions?
...and more on that point, I drive an RX-7 and my engine puts out _extreme_ amounts of heat under the hood. a 50C top-end probably isn't enough for driving on hot days.
I would go for one of these without a second thought because a) though my car is light, reducing more weight helps out loads with performance and b) these things are known to kill alternators and I'd love advance warning of when I might end up stranded without having to test the battery's volts every day.
I'd love to demo one of these.
Still would want a lighter battery though.
This picture shows a 10x decrease in life when the temp varies from 25C to 65C: https://en.wikipedia.org/wiki/File:EDLC-Life_time_curves.png
[ 1 ] https://en.wikipedia.org/wiki/Supercapacitor
EDIT: I'm not trying to inject bad feelings here or contradict Ohm. I'm just clarifying the effect of temperature on supercap life. There is some effect on cap lifetime at high temps. I hope the product succeeds. I know sailors have using lithium batteries for a while now, but as mentioned elsewhere, they are shockingly expensive (like $6k).