I've driven a car with a manual speed limiter for 10+ years now. I don't understand why all cars don't have one. Entering a 30mph/50kmh zone? Set that as the maximum speed on the limiter and you can drive around normally without having to keep checking your speed. Less time checking your speed equals more time looking where you are going. This is only a good thing.
I completely disagree. If there is roadway traffic, you don't need to check your speedometer as the safest and smoothest thing to do is to simply travel with the traffic. If you drive much faster than the bulk traffic flow, you risk causing an accident. If you drive much slower than the traffic, you risk getting rear-ended or clipped as irritated drivers people pass you. You also substantially disrupt traffic flow and actually slow down everyone's commute.
If there is no roadway traffic, it really shouldn't be much of a burden to check your speedometer every 5 seconds, preferably right after you scan your mirrors.
Devices like this actually promote driver inattention and complacency. People get bored and their minds wander. Or they think that it allows them to spend more time on the phone. All this new feature is going to do is result in some idiot driving at the speed limit, in the passing lane, exactly matching the speed of the car right next to them and perfectly blocking all traffic, while dicking with their cell phone while 2000 irritated drivers piling up behind them.