I have a Snooper S4 and it works a treat, even with the Heat Reflective Windscreen that is fitted to my 172.. So no need for the optional Re-radiating antenna..
On the legality front the GPS driven "detectors" are totally legal, and this will remain to be correct..
However a point to be aware of is that there is a bill due to go through parliment that will very likely make "Radar" detectors illegal again..
This will include the "radar" only units and the plug in optional Radar detector heads that are available for most of the GPS type units..