I've tried the i-Stop on the Mz6. It's actually rather foolproof and not intrusive. It was smart enough to know when I was in stop and go traffic and when I was really stopped because of a red light, and it only turned the engine off during the latter. Aircon also remained on despite the engine being off, though it was much weaker (similar to when you turn your aircon on with a conventional car placed in ACC mode).