We have this in our new Kia Sorento, and I'm currently experimenting with it. I don't really have anything against these systems. Having the engine shut down automatically when stopped seems fine to me.
But I can't work out the implementation in the Kia. The car is an automatic, and it seems to shutdown the engine if you come to a stop for a certain period of time, with your foot on the brake. When you lift your foot off the brake, it restarts. But there's been a couple of times where I've been stopped at a junction with my foot on the brake, and then just as I was about to decide to pull out, the engine shut itself off - I literally lifted my foot off the brake at the exact second the stop/start system cut in. There was then a delay while it fired up again, which really wasn't helpful as I was just committing to pulling out of the junction. I feel like it would actually be better with a manual gearbox, so it would only shut down once the car was in the neutral and the clutch was up.
To add to the confusion, the car also has an "auto hold" option, where pressing the brake firmly locks the brakes on, so you can take your foot off the pedal and not creep. But this function has to be enabled each time you switch the engine on. It also seems to take only a slight increase in pedal pressure to activate, and I'm not sure if this also triggers the stop/start. What I'd really like is for the "auto hold" to be something which is always enabled, and which I activate by pressing the brake pedal much more firmly, and then it also triggers stop/start. That way, if I'm waiting at a junction with light brake pressure I know it wont shut down - whereas if I'm waiting at lights, I can press the brakes more firmly to have the car hold itself and shut down until I accelerate gain.
If anyone else with a Kia/Hyundai knows more about how the systems work, please shout (or perhaps I should just RTFM).