As I travel regularly in France for prolonged periods of time regularly I needed to find a way of getting rid of roaming charges with my phones and my iPhone especially and it's data connection. Therefore I've done extremely extensive research on the market and thought I'd share my findings with you. I've separated this into two sections: calls&texts and data to make it clearer depending on your phone usage (ie nokia vs iPhone). Also SIMs are not free in France so I've put their cost and the current offer (if any) which come with it.
Calls & Texts
Simyo is the cheapest for calls and texts in the country 0,19euro/min calls and 0,10euro/text you and topping up 10 euros will be valid for 3 months and your line stays active for 6 months after you run out of credit. (I can send you an invite to get an extra 5euros credit for free). SIM: 4,90euros - you get 5€ + 5€ extra free credit
Aside simyo, zeroforfait has pretty much the same call rates ( 0,19euro/min , 0,085euro/text) but you have to spend more than 15 euros/3 months to keep the line active so if your a slightly bigger caller and you intend to stay for a prolonged period of time this can be a very attractive offer too. You also get to choose 5 numbers which you will have half price call rates on weekends. On top of it they have the lowest /Mo data pricing in France=> 0,59euro/Mo, they also offer a 250 MB data package for 17,90 euro/month.
SFR have recently launched new top ups designed for the iPhone: 3€ for a day(20MB FUP) , 10€ for a week (150MB FUP), 20€ for 20 days (500MB FUP) or 24€ for 20 days data (500MB FUP) with 10€ of credit for calls and texts. Line stays active 6 months after expiry of credit. The 24€ top up is a really good package as it brings down the price of calls on the SFR network which are usually 0,55euro:min for calls and 0,12euros/text to 0,22euro/min and 0,048euro/text which is pretty close to the cheapest calls and texts offers I detailed previously thanks to only paying 4€ for a 10€ top up. SIM: 15-30€ depending on retail point
Bouygues Telecom have a data only offer for mobile broadband use which is quite good. no data use-> 0€/day, 2€/day when you use less than 20MB data and 8€/day for unlimited data for the day. All of this is fully automatic and very flexible. Obviously it is more costly to use in an iPhone than the SFR offer but it has the big advantage of being on the bouygues network which has a much more developped 2G network than SFR has so you have good data coverage in areas where the SFR or oange don't have much if any - obviously that's dependent on your location. SIM+USB stick: online: 9€ - you get 24€ free credit until the 17/01/2010
I will note that the prices I refer to are the ones on the date of publication for national calls only, they probably offer the cheapest international calls on mobiles but I haven't checked and home broadband in France have very good unlimited calls to international included in the average packages. I have not checked how simple the payment process is internationally (ie can you use an international debit/credit card to pay), what I know is that SFR only requires a photocopy of an official EU ID (passport,...) and you can pay cash so pretty easy to pick up.
Also if you contact me to get a simyo invite to get 5€ extra free credit I will also receive 5€ free credit by simyo.