I turn it on. There are two long traffic lights on my commute, and I like having the engine shut off while I'm waiting. I can either see the light change or have to wait for the other cars to move, so I have plenty of time to release the brake for the engine to fire back up. The battery will probably get replaced before the warranty is up, so I'm not concerned about that. I'm not always attacking the streets, so some times it's nice to safe a little fuel.
Current: '13 E92 M3 Euro Delivery 9/4
Wife's: E39 M5, E36 M3, E30 M3
Gone: E86 M Coupe