Where the stop-start system makes sense is in stop-and-go rush hour driving.
Back when I was a poor student commuting to school, usually running on fumes, I'd routinely shut my engine down when the traffic stopped. It really bites to run out of gas in the center lane of an interstate, with long lines of cars on either side of you. Stopping the engine during prolonged idling can indeed save significant fuel.
Far as normal driving? I'm not convinced. 1) The starter motors in cars so equipped are insanely expensive compared to the old school units; 2) there's just that much more stuff to fail in control circuits etc.; and 3) while driving a rental not too long ago, I would have been smashed flat by a semi had I not disabled the system, allowing me to nail the throttle to get out of the way without having to wait for re-start.
Hundredths of a second can make the difference between life and death, and I'm not the only one who's experienced similar almost-disasters.
Some cars now do not allow for the system to be disabled, so pay attention.