According to this site, flywheels can have a greater energy density than batteries. Also, no problems with chemical deterioration or memory, etc. Their smallest unit is 300 lbs and can produce 50KW for 20 seconds, so it's ideal for high power, short duration applications. Their big model is 500 lbs, turns up to 48,800 RPM and can supply 750 KW for 3 seconds - that's a lot of juice! It's only rated at 25 to 140 deg F, but this is aimed at Earth-bound commercial applications.

RPM claims thier units can supply output for days, and can be on standby for YEARS without significant losses.

Cool stuff! Lots of info out there.