While in Vegas last week, I took a 30 minute test drive in the new 2011 Porsche Cayenne S. It had the engine stop feature (could be easily turned off/on). With it on an getting caught at the notoriously long traffic lights around Vegas, every time I came to a full stop, the engine turned off. But every time it turned off, it felt like the Cayenne got tapped from behind by another vehicle. It did restart as soon as I moved my foot from the brake pedal. It also would start while still stopped when the vehicle sensed that the a/c needed the compressor to be run by the engine.
If I had this feature on my M3, or any vehicle for that matter, I don't think I would use it because of the above tap from behind nuisance. Maybe I would get used to it and maybe with the DCT, the M3 wouldn't give the feeling like it just got tapped by another vehicle.