I'm on Sprint and I've never had an issue. Usually I just use it for browsing when I am on the road but once I was without an ISP for a month and I did something like 9gb of transfer. They never said anything but it was just the one time. I had downloaded a few big torrents I think
Either way I've not had any issues and when I just use it for browsing on the Amtrak or whatever it doesn't rack up too much.