I have a grandfathered 12/1 plan and the usage meter shows a cap of 250GB. Last month I used 612GB and there have been no extra charges.
I do question the accuracy of the meter though. I didn't use the internet any more than normal but some how used 600gb of data. The month before shows way under my normal usage. This month I was at my the meter shows I've used 300gb in 4 days. That can't be right, at this rate I'll be at 2TB by the end of the month.