The biggest "complaint" with auto stop/start in BMW's is that it has to crank the motor with the starter when you lift your foot off the brake. And it's pretty "violent" compared to auto stop/start people have experienced in hybrids which use the electric motor to easily restart the motor.
From the MB website:
The engine goes off when the vehicle is stationary and back on again when the brake is released – the ECO start/stop system operates virtually unnoticed by the driver. This is all thanks to complex control functions which run in the background to ensure efficiency, comfort and safety.
How the ECO start/stop system works in detail: as a general principle, the engine is switched off every time the vehicle comes to a standstill (auto-stop function. Restarting the engine (auto-starting function) takes place almost imperceptibly; and moving off without any noticeable time lag compared to a stationary vehicle with its engine running. This rapid action is attributable to two special features: a crankshaft Hall sensor which is able to identify the rotational direction of the crankshaft enables the engine control unit to identify the cylinder in which the piston is ideally positioned for starting. The fuel is injected into this cylinder first, thereby speeding up the starting process. An additional electric transmission oil pump supplies the clutches of the automatic transmission with oil pressure prior to starting, so as to enable a swift resumption of the journey after direct starting of the combustion engine via firing of the spark plug on the ideally positioned cylinder. The starter motor engages briefly, imperceptibly, to complete the auto-start function. The starter motor (starter) has also undergone thorough modification: it is now designed to cope with eight times as many starting procedures, ensuring that it will last a car's lifetime in continuous urban driving involving frequent auto-starting. In addition, the on-board electrical system is supported by a second battery.