I've found the blind spot monitoring as implemented by Tesla utilizing the ultrasonic sensors to be useless.
Contrast this with the blind spot monitoring system as implemented by Volvo on my wife's XC60 using radar emitters, one on each rear corner of the car, to be 100% reliable and effective, and low latency.
Also, the Volvo has indicator lights on each A pillar base, near the rear view mirrors, that are much more noticeable than the near invisible curved lines on the Model S dash display.
The rear radars also allow for another really useful feature - when backing out of a parking space, the Volvo looks down the parking lot both directions with the radars and warns if a car is coming your way. Really great when you're stuck next to a big SUV or pickup and can't see down the aisle when backing out.
I hope Tesla revs the hardware, or pulls some magic software tricks out of their sleeves.