Them flying Gendarmes and customs people will have a go at foreign based pilots as well as French based ones - they need to justify their existence. I am a French based, French pilot flying....on a UK CAA PPL. I have had to show them my ID and other documents only twice but I did have to teach them a bit of French law, reminding them that any pilot with a licence issued by an EEC country can fly French reg. aicrafts.
Another interesting one happened in Dinard where they would not let me go to the loo in the terminal because I did not have my passport (I had flown from down the road!).
In general, France is a hassle free place to fly.