For travel to US from UK or for someone living in US?
If travelling, be aware that most UK operators charge extortionate roaming data fees for North America (Orange/EE – £8 per MB !!!! and their bundle packages are pretty rubbish also). Three I think are one of the few that’s any good for roaming in US.
Travelling to US, turn data off and use wifi where you can. Careful with calls and texts also (£1+ per minute etc and often the same charges for receiving calls).