Accident Prevention Devices on Cars; Any Good?

I notice in some car ads they are increasingly promoting accident prevention devices (though they are mostly offered on luxury cars and the higher trim models of non luxury cars). They have things like blind spot sensors, back up protection etc. There is one ad (I forget which car) where the car detects an impending accident two cars ahead.

Does anybody have any experiences with these devices? How useful are they?

One of my friends owns a new Audi which has a collision detection/avoidance device. He told me that he was backing out of his garage one day when - just as he noticed that he was about to hit his wife’s car - his Audi sounded an alarm and hit the brakes on its own.

He believes that he would have been able to hit the brakes in time to not hit his wife’s car, but he’s happy not to have had to put that particular situation to a test.