I visited France last month for about 25 days and used the Orange data network for calls and data for an iPhone 4, using a prepaid Mobicarte. Coverage was excellent and data speeds were usually at least as good as I usually get with Telstra in Australia. Congestion did sometimes occur in busy areas but was not too bad. I was also in a lot of rural areas and was pleasantly surprised by how often I could get 3G; if 3G wasn't available it was only in extremely remote places that Edge was not available.
I subscribed to Internet Max (500MB) for 9 euros and Mail for 6 euros. This meant that once these had been deducted I only had to worry about the balance for call charges (I hardly made any) and had effectively unlimited surfing and proper mail rather than webmail. VPNs worked fine as did video streaming. I didn't try Skype calling as the terms and conditions specifically prohibit it.
I had internet from the time the plane taxied in at CDG until I left the country. And no, I don't have a French credit card or a friend in France to buy the SIM card for me. Here's how to do it:
1. Buy a prepaid, pre-activated Orange Mobicarte SIM card on Ebay. There is a seller with about 2000 positive feedback - he is legit. Ensure you buy the one with 15 euros credit. The credit expires within a month so don't buy it too early before travel.
2. Turn data off and put the SIM in your phone in your home country (you will have to cut the SIM to micro-SIM size for an iPhone4). You may have to try a few different local carriers before you get one that will allow the SIM to work on its network.
3. Log on to the "Portail Orange" website
Portail Orange : Actu, Sport, Assistance Internet, Web Mail Orange (French only) and create a user account using the phone number. To get the password you will have to send a SMS to the phone from the website - this is why you need the phone turned on. The SMS will cost 50 euro cents or so.
4. Once you receive the SMS, finish setting up your account on the Portail Orange. At least two or three days before you travel, activate the Internet Max Option - it takes up to 48 hours to come on line. You will not have enough credit left to activate the Mail option yet.
5. For an iPhone, sync with iTunes with the SIM card in before you leave home - this will ensure that the correct carrier settings for the SIM card are loaded.
6. Once you arrive in France you will have phone and internet available immediately. You can buy 5 or 10 euro recharges at any newsagent so that you have some credit for calls. You can recharge and also activate the Mail option through the #123 phone menu (French only).
Benefits:
No wasting time in France buying the SIM card and waiting 48 hours for activation;
Know your phone number in advance to tell friends and family.
No messing around with carrier settings in France - get it all set up before you leave.