100% agree, brilliant devices that change the way you ride. I had to show a policeman recently who honestly didn't believe me when i said that my radar device detected upcoming cars, he thought i was joking until one went past.
I never leave home without mine for road cycling but agree in town it would be quite useless as it would just keep going off.
Up to 5 seconds before a car is in earshot i know it's there and can be prepared ie not change handlebar position, avoid drinking, checking out the road ahead to see if there's any reason i may need to adjust my route (pothole) and be ready.
It's the one bit of cycling technology that really is worth it