I have encountered several situations in France: on highway toll booths most Visa or MasterCard work, amex is spotty. In stores, Amex is not accepted a lot of times, if it is, they typically want a signature. Visa and mc are pretty much universally accepted, a lot of times the cashier doesn't know what to do with the signature slip that is generated and just throws it away. Some make you sign. At some gas stations none of my US cards worked. So, YMMV.