I can tell you that they are worth while and worthless at the same time.
Before I became and Investigator, I worked traffic for 3 years. On the interstate, radar detectors never really kept me from catching people because I used Lidar (laser). The laser wouldn't trip a detector until I fired it at them but by then it was too late. I nailed numerous folks 110 + on bikes and cars a like that had detectors.
Now, in town or in the country where the road curved or went up and down over hills, the detectors absolutely gave people warning. Radar is more efective in these situations and the detectors could detect the radar even if it wasn't firing (passive). There were instances where I could here a bike coming, obviously hauling a#$, but by the time he was visible he would be in the legal (under 20 over) range.
I don't use one but I would say that they deffinantly save people money.