My experience is with a manuel Jeep, others may vary. It saves fuel when driving in the city. Engine stops when you stop, starts back up again when you push in the clutch. Other than being unusually quiet when stopped at a long light, it’s completely unobtrusive from a driving perspective.
It doesn’t stop if the engine isn’t up to temp, the wheels are turned, you’re on too steep of a hill, or if the interior temp gets too hot or cold.
It’s a good thing and I don’t understand why people whine about it so much.