Getting a SIM card for anything besides voice calls is extremely difficult and expensive in France and may take up to 48 hours for activation. A better alternative is to use roaming on a UK SIM under the new EU “Euroaming” system. Out of the two major telecom options in the country (SFR and Orange), SFR is comparatively easier to deal with. For Voice & Data plans, SFR is roughly 33% cheaper then Orange. Another thing to remember is that the nanoSIMs for the new iPhone 5 are not available in France.
Find more valuable telecom advice for travelers to France at:
The best SIM cards for voice and mobile Internet data in France