JAD79,
Just to be clear you need the 3G version of the Ipad but you DO NOT need another data plan as your Ipad will use the Personal Hotspot from your Iphone when required. In flight you do not need the Hotspot facility or 3G for the GPS to work. I tried mine recently at FL270 and the Ground speed readout was identical to the FMS. Note that the altitude readout is true altitude so there is a large difference to standard FL altitude at 1013.2 MB. This however is a good feature when close to the ground in my opinion.
Groggy