We have established that GM knows how to make a two-mode hybrid. Witness the Green Car of the Year*, the Chevy Tahoe Hybrid, which utilizes a powerful electric motor in concert with an internal combustion engine. Still, GM continues to build its "mild" hybrids, using the belt-alternator-starter system, yielding in the Chevy Malibu a grand total of 2 mpg in city/highway fuel economy over the standard four-cylinder. This option costs a whopping $1,795 and could only be worthwhile if buyers put in for the $1,300 federal tax credit.
To compare, a Toyota Camry Hybrid option costs $200 (over the four-cylinder XLE) and yields an increased fuel economy of 31%.
If I may quote myself from last year, GM's BAS technology is a maximally minimal effort that's more about affixing "Hybrid" badges to cars than improving fuel economy. The salient advantage of the BAS system is that the gas engine is able to shut down at stops, but only if the air conditioning isn't on. In fact, in my two days in the Malibu Hybrid, the engine rarely cut off at stops, apparently because of the state-of-charge demands of the nickel-metal hydride battery. Meanwhile, the electric boost offered during hard acceleration amounts to about 5 hp, which is imperceptible.
My suggestion? BAS technology is not a bad thing; in fact, all cars should shut down their engines at stoplights, and run their accessories (steering, AC) on electrons. But this should be a government-mandated feature, not a pricey addition.