I own a PHEV (chevy volt) and it's terrific, it behaves like a stop start when running on gas. Starting the car out in electric then using the secondary electric motor to star the engine seemlessly (to which then the engine turns the motor as a generator most of the time) no delay, no sudden starter noises and smooth movement while at a light or in traffic.

I'm seriously suprised more automakers haven't adopted some form of a hybrid assist drive instead opting for beefy starter motors and stop/start to reach EPA goals. Even a limited capacity battery with a smaller electric motor would help MPG substantially being able to assist the motor from a stop, and act as a generator helping recharge that battery while saving your mechanical brakes from excessive wear when slowing down.

The savings in stop start certainly lend to specific scenerios where it's use case is most efficient... Which likely isn't freeway commuters with lower traffic that actually stops often.