I had auto start/stop on the 535i I drove in Europe. I must admit at first it was a bit creepy having the car go dead silent at the lights, but I quickly got used to it and actually rather liked it. The car sprung to life pretty damned quickly as soon as you let off the brake and got ready to hit the gas pedal... but I will warn you that jackrabbit starts from the lights are a no-no. I got hung up once or twice by the car being half a heartbeat behind me when trying to get around a left turn at a set of lights... sometimes quite scary when a truck is heading right for you.
Now, having said that after 10 days in Europe I was well accustomed to it and found it nice and relaxing to have the car shut off if I was at the lights long enough. It does get around the battery discharge problem because if the battery is too discharged it won't shut the engine off. Similarly if you're sitting at the lights for a REALLY extended time then the engine will fire up periodically to keep the battery charged.
The only real downside I found was that in combination with ComfortAccess it can confuse you a little. Once at a rest stop in Northern Germany I returned to the car after a few minutes to find it running. I had neglected to hit the stop button when I got out, and while I had locked the car, when I got out I didn't think about the engine because I didn't hear it running. Because it wasn't running, of course. I laughed, but I couldn't help but be a bit creeped out by the idea that if I had stopped in a bad neighbourhood and not locked it, someone could easily have driven away with the car even without the key.
2001 E46 BMW 330i Sport Package 6MT
2004 BMW R1150GS
2004 E60 BMW 545i Sport 6MT
2012 BMW 135i - Deep Sea Blue / Terracotta / DCT
2012 BMW K1600GT - Vermillion Red Metallic
2008 BMW E90 M3 - Interlagos Blue / Silver / 6MT