This weekend I am in Paris for a nice long weekend, including Disneyland. This is the second time I am using a sim-card from LeFrench Mobile to call a bit cheaper, especially local, and use mobile data. The service could be a bit cheaper, but is still cheap enough. Coverage is good, in the Paris area, and data works well, although you have to download a .mobileconfig for the iPhone. The best thing is that the website and all the other information is in English, instead from all the other operators.
I got mine through SimUp, within a day, other countries are available.