Mr. Bradley Berman, editor of hybridcars.com, wrote:
http://biz.yahoo.com/weekend/hybridmyth_1.html

<<Auto engineers have developed an ingenious system known as regenerative braking. Energy usually lost when a vehicle is slowing down or stopping is reclaimed and routed to the hybrid's rechargeable batteries. The process is automatic, so no special requirements are placed on the driver.>>

Actually, the batteries are charged most of the time by the electric motor, powered by the gas engine and functioning as a generator.
Only about 30% of the braking energy is regenerated. The rest is wasted as heat. Regenerating the braking energy accounts for perhaps 10% of the total used to recharge the batteries, unless one is driving downhill and braking most the time.