While we were travelling through France to attend Drupal Developer Days Montpellier 2015 we needed a 4G data plan to connect our several gadgets. Getting it in France, surprisingly was way more complicating getting the SIM card than last year in Hungary.
After some research (and help from our European friends) we decided to settle with the Orange 4G "la pochette prêt-à-surfer". You pay 9.90€ for the SIM Card and 500MB bandwidth, which is more than what we needed for one week of event.
Please notice that you must have an unblocked phone and you may need to manually configure your smartphone APN as well.