France has possibly the worst prepaid plans available anywhere in Europe, due to lack of competition. Most of the mobile operators allow you only to buy data in small blocks measured by the hour, rather than by day or by the amount of data that you want.
They also have an intense dislike of tethering, and you may find yourself losing all your credit, if you do so.
Hopefully this situation will change when a fourth mobile operator starts in 2012.
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