I've had one or similar for over a year. I think they are as good as any but...
They are good at telling you have gone past one or that there is one on the other side of the road. Not so good at telling you you are approaching on but do give you a bit of a warning.
the false alarms mean you ignore it sometimes when you shouldn't!
They are not good at hand held lasers (like Police use on bridges etc as the laser guns register your speed in a split second. Also they are not good more recent cameras which face you which seem to be triggered by wheels passing over a sensor.
All in all they are OK but a long way from fallible. I did see one based on GPS advertised - it knows where you and all the cameras are and you get updates via the web.
Bit pricey though