We did seven weeks in Normandy and Brittany with our vodafone dongle. Hit lots of SFR free wifi spots but couldn't get in. They are for SFR customers, strangely BT could connect but not Vodafone who are SFR partners.
Tried to buy a SFR PAYG but they wouldn't sell me one because I didn't have a French Bank Account. VISA or Cash was not acceptable.
John