Decent but limited app
If you mount your iPhone on your handlebars, you get a nice live map showing detected cars relative to you. (It only supports portrait/vertical display, FYI.)
But except for mute/unmute, there is no control over the sounds or their volume. That really matters if you don’t have the visuals from a bike computer or a handlebar-mounted iPhone.
The alert tone is fairly audible when your phone is in a pocket or saddlebag, but the all-clear tone is not audible unless you are not moving— it takes very little road or wind noise to drown it out. The alert tone is more audible but can also get drowned out by strong road and wind noise.
Control over alert volume and choices over the tones themselves are needed so you can get the best audio based on your ride conditions and your own hearing (what you can hear changes as you get older!).
There is no Apple Watch support to make the radar status more accessible when you have no mounted phone or bike computer.
Because the CarBack hangs on to the Bluetooth connection with your phone even after you turn it off with its physical button, you can get surprise alerts that the CarBack is no longer detected long after you turned it off. This seems to be antitheft feature, such as for when you park your bike for a break during a ride, but you can also get alerted at home when you are done with your ride and are no longer in range. This feature needs more thought.