Well, volvo now has a prototype 'bile, wherein the flywheel powers the rear wheels and a supercharged four cylinder ICE the front wheels and the flywheel. Of course, the flywheel is used for regenerative braking, also. Note: the flywheel is vacuum packed and requires 30 minutes to decay. I suspect the system is not as "good" as the battery methods, but may be somewhat less expensive.
Volvo S60 Flywheel KERS Prototype First Drive - Motor Trend