Until recently, the AMEX acceptance in France was a joke. Only hotels and some top-end restaurants/shops were accepting it (and even then mainly in the touristy areas like Paris or the Cote D'Azur). On my recent trips to France I have noticed though that there are more and more places you can pay with the card, even outside the main tourist areas....
Having said that, I do have to admit that I still have to get the Mastercard/Visa out in a lot of places. Also on couple of occasions, I have been asked if I did not have another mean of payment (even in restaurants where the AMEX sign was clearly present at the door and the bill holders and/or Reserved table signs were labelled AMEX) as the comissions they have to pay AMEX seem to be huge in France.

-jani