IEEE Spectrum reports: "Because no chemical reaction is involved, ultracapacitors—also known as supercapacitors and double-layer capacitors—are much more effective at rapid, regenerative energy storage than chemical batteries are. What's more, rechargeable batteries usually degrade within a few thousand charge-discharge cycles. In a given year, a light-rail vehicle might go through as many as 300 000 charging cycles, which is far more than a battery can handle. (Although flywheel energy-storage systems can be used to get around that difficulty, a heavy and complicated transmission system is needed to transfer the energy.)"
Every year I expect better battery technology to be developed for all the new electronics. And every year I am pretty consistently disappointed. The ultra-capacitors are where I would be putting my money, rather then on the older chemical based battery technologies.