Legislation in France banning the use of these devices (including Sat Nav if enabled to detect the location of speed cameras) became effective on 3rd January 2012. It is important to note that the offence is commited whether or not you are actually using the device; having it in the vehicle is the offence. At the least, you should disable the facility but it may be that even that is not good enough (yet to be tested). French traffic police have the power to issue on the spot fines which can be up to a maximum of 1500 euros - and you may get your vehicle impounded too.
Incidentally, closer to home, the 2006 Road Safety Act contained a provision to allow the Secretary of State to make similar regulations here but so far this provision has not been activated. Happy travels!