Never use it on the M3. Was fully aware of the gas mileage and was not a consideration when buying...
I did have my car in for routine maintenance today and the F30 328 loaner had auto start/stop activated so I left it on as I ran errands. Not a terrible feature as it seemed minimally invasive in city traffic, at least on the 328. Still, don't see myself using it on the M3.
The one thought that kept crossing my mind was the impact on the battery. These car already have battery issues related to insufficient charge related to start and stop driving patterns and short trips. I have to believe the start/stop feature would only make this situation worse with the starting and stopping in city. No proof to back that up just a concern based on my limited exprerience.
2011 Space Grey E90 M3, DCT, Comp. Pkg.
1997 Z3 2.8, Artic Silver, Tanin Red interior, MT
1974 2002tii, Inka, 5 sp manual