Even though it's written on AT&T's site about unlimited data, it's not if you read the terms:
AT&T is always one problem after another. I probably use less than 1GB of data every month, though.
If you are on a data plan that does not include a monthly megabyte allowance and additional data usage rates, the parties agree that AT&T has the right to impose additional charges if you use more than 5 GB in a month. Prior to the imposition of any additional charges, AT&T shall provide you with notice and you shall have the right to terminate your service.
AT&T is always one problem after another. I probably use less than 1GB of data every month, though.