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How do I use my iPad in another country?do I just buy a French sim card, put it in and go?
I notice you are from the UK and you mention you are travelling to France. You should have no problems, as the other posters have suggested. Your UK SIM card will work in France but you may need to contact your service provider first of all to have 'roaming' enabled. If you have a 'monthly' contract, then your roaming charges will be in addition to this monthly contract. If you have a PAYG (pay as you go - i.e. pre-paid) service then the roaming charges will simply be deducted from the pre-paid sum that you have available 'on' the card.
As has been suggested, roaming charges could be an issue, though the EU has recently imposed limits on roaming charges that service providers are allowed to impose.
So, as you hinted, purchasing a French PAYG card could be a good option. Depending on the card that you purchase (of course, it needs to be a micro-SIM) you may need to set up a few service provider specific parameters in the iPad's Setting app - such as the APN, user name and password - although most service providers/SIM cards do this automatically.
As one of the posters pointed out, though, WiFi is very widely available in France - as in the UK - so most malls, hotels, franchises, airports, train stations, trains, some buses, coffee shops, restaurants have WiFi - sometimes free too. So, if your stay in France is mainly going to be in urban areas or in hotels, you may not really feel a need to use your 3G at all.
Whatever you decide, I hope you have a great vacation and enjoy your iPad in France.
Tim
Scotland