I have a co worker on AT&T who uses MyWi. He'd been using it for a long time however he let his kids use it to watch Netflix on a long road trip. Totally spiked his data and sent a red flag to AT&T. They charged him for the data and placed him on a different plan. He called and some how talk them into removing the charges and restore his unlimited data. He promised not to do it again...
Just a story I thought I'd share. Is it helpful? Probably not lol. But I do know some people with AT&T and Verizon (myself) who use it and don't go crazy with it and are just fine.