The product worked absolutely fine. However, please be aware that although you can register the sim on line and get a US phone number before you get out, any online top ups are not possible until you get to the US.
As soon as I got to the US the phone registered without any problem. Once registered, you can ask for an online access pin, which will be sent to your mobile, and then you can top up on line. You can also top up using 611 or 1 800 901 9878. ATT as a matter of course don't process European credit cards. You will be put through to a specialist credit team. They will also try and talk you into a $25 top up and to sign up for automated top up. You can always decline their kind offers.
Definitely worth having if you're in the US for any period of time.