I have the Bel electronics unit. It has paid for itself many many times.
If there are cars in front of you, it will let you know when those cars were getting read. If you are alone on the road, you are screwed and it will only tell you that you are getting a ticket.
It works especially well in the desert, where there is nothing, and then it goes off on a car two- four miles down the road.
They decrease your chances of getting a ticket, but if you are the only person they click the radar on, you are screwed.