Unfortunately it's not a good idea Dave.
They are illegal in France. Not only do you get hit by a large fine, they will simply rip the system out of your car and don't care how much damage they do in doing so (i.e. if you've wired it in behind the dash, expect a damaged dashboard.
To add insult to injury, it is quite common for the Gendarmes to then place the unit under your wheel and make you drive over it!
That said, this is what happens to British cars with them. Perhaps Gilles may know if they are perhaps more lenient to French plated cars??
I lived in France for 2 years... I have what I call my "Wall of Shame", which is where I framed my "souvenirs" of my time in France that were given to me by the Gendarmes... So I have some first-hand experience with speeding and tickets ;-) All those were in a car with French plates.
I later bought a radar detector in the US and used it quite successfully in France and Spain for a while. It was like shooting fish in a barrel - since detectors are illegal, they don't use "instant on"... They set the radar guns up on a tripod and hide behind a bridge or something with the radar gun pinging away. My detector helped save me from several more tickets.
That was all in the "good old days", 3-4 years ago. What has been written here is correct - you get in far more trouble today for being in possession of a detector than for speeding so I haven't used mine in years.
The French have also started to get more serious about speeding. I can't tell you what the threshold is for issuing tickets in France, since mine were for 30-50 kph over the limit. At that time, getting nicked meant paying a 60-90 euro fine and nothing more... with my US license they couldn't issue me points and it was no big deal (which is why I drove how I drove). The fines have recently been raised substantially. I would guess that you could probably cruise at 145-150 kph and not have too many problems?
Since you are coming over from the US you should know that the method is completely different than they use over there. There are very few (no?) rolling radar traps... In the US, Smoky cruises along the interstate and occasionally shoots oncoming traffic to check their speed and may do a u-turn to go get the offender. In France they are all stationary radar traps. Generally after a bridge or building or some object that they may park behind. They then radio ahead to their buddies your license plate number and the kind gentlemen will wave you over to the side of the road, or will be waiting for you at the toll station. You will then have a short chat and they will relieve you of any excess cash you may have. Perhaps 25 days of the month there will be zero speed enforcement, but on the 3rd, the 11th, the 18th and the 27th they will set up one of their radar operations and they'll do a brisk business on those days. They have also recently begun to set up photo radar but they aren't as widespread as other countries yet.
Hope this helps
