In my Hyundai, it's only enabled when certain conditions are met, like temperature above a certain level, battery charged and only after you've driven the car for a few minutes. In addition you can manually disable it.

The main benefit I got from byuing a car featuring what Hyundai calls ISG was a lower tax on the car, as here in Denmark that is dependent on the unrealistic manufacturer reported fuel efficiency. In real life I find the effect of the start-stop system is negligible.