Wikipedia has a nice article on the technology here:
Start-stop system - Wikipedia, the free encyclopedia
I really like the Mazda system: "The i-Stop system, Mazda’s first start stop system, detects which piston is in the best position to restart quickest, which is the one in the combustion stroke phase, where air and fuel are in the cylinder, ready to be ignited. The mixture in this cylinder is ignited by the spark plug, forcing that piston down, resulting in a near instantaneous engine start time of 0.35 seconds.[16]"
They list the following as issues: "Many people think that long term use may induce additional wear due to lack of oil lubrication, but this is not true. In a start stop system the short stop times keep the engine and oil warm, retaining lubrication.[23] Some implementations do not use a starter motor, eliminating concerns of starter motor wear. Mazda i-stop used in their Mazda3/Axela line (in Europe and JDM) uses combustion to restart their engine by sensing the position of the piston in the cylinder. They claim quieter and quicker engine restart within 0.35 seconds.[24]"