I've used the Mercedes system. Their cars beep constantly. I'm too close (2-3m) to a car ahead that's turning away, beep beep beep. Ignition before seatbelt, beep beep beep beep beep. Signalling intention to turn in traffic, beep beep beep beep. Driverside front tire 3psi low, beep beep beep. Passenger side tires blown, beep beep beep.
The beep means nothing. It's far more distracting thinking the tires are gone looking down to read the message, when it means to say a car is too close.
It's a very good idea, with poor implementation. They're working on Active prevention of accidents using the system, where the car won't let you change lanes. Instead of the system acting like a road-phobic passenger, it'll be like them grabbing the wheel. I love the idea of driver assistance and driverless systems, but they don't know context, and interfering with the driver can cause accidents.
Very interesting feature for the future, but a distracting toy now. If a car has such terrible side visibility to need this feature, I'd consider it an unsafe car. If other drivers try living with it and find it helps them a lot, then I hope there is an aftermarket kit for them.