I use the feature quite regularly when I can. The "when I can" part was added because the BMW engineers in their infinite wisdom tie start/stop function to the icing/snowflake function (37 degrees F), even with a fully warmed up engine, so that means in Chicago it's basically unusable December to March (even though the highs occasionally get above 37, for typical 8am/6pm commute times, it's rare). Couple that with shutoff of the feature when it's too hot (I think it's somewhere in the 70-80s), I figure I only get 50% usage of the feature.
That said, there's obviously a lot of tech that goes into deciding when it engages, so you don't need to worry about it auto-stopping a cold engine for instance, and the starter has apparently been uprated for the additional usage. Also, this feature has been in use in Europe for far longer than it's been showing up in American cars (other than hybrids), and if it was causing issues, they'd know by now and correct or stop using the feature.
The value of auto start/stop definitely depends on your commute. For mostly highway drivers, it's pretty much worthless, but if you have a city commute with some long lights, it will absolutely save some gas. It also seems to be very well integrated with DCT, but might be more annoying with 6MT. It will only stop the engine and stay stopped if you keep your foot on the brake, which makes it easy to alter your behavior based on whether you think you'll be at the light long. If you are, keep braking; if not, you can brake enough to stop your car then take your foot off the brake. In the DCT, the car won't move, and the engine will never stop. Also, if your light is getting "stale", or you see the opposing light turning red, take your foot off the brake, and the car restarts. Even in instances where I haven't "prestarted" the car, it restarts so quickly that I've never had much of a delay from the restart.
In short, I find the feature to be very useful at times, albiet limited by what I view as a serious flaw of shutting it off at 37 (I think it could easily be lower, as long as the engine is warmed up), which seriously limits its applicability in colder climates, and probably limits real world fuel savings to far below estimated.
2011 Jerez Black/Fox Red E90 M3 DCT, ZCP, ZCV, ZCW, ZP2, BMW Apps
2004 VW R32 (track car)