I have the Nexus 7 3G, and activated the AT&T Micro SIM that came along with it. I picked the no-contract $30/month 3GB plan. Just tested it for a day so far, and speeds & coverage seems fair at 2-4Mbps down.
As much as I despise AT&T, that plan seems better than what T-Mobile offers specially for tablets & internet devices:
$25/month - 1.5GB
$35/month - 3.5GB
After this month expires, I'm going to order a T-Mobile Micro SIM, and see if I can activate that no-contract $30/month plan with unlimited text & data (throttled after 5GB) and 100 talk minutes. Not sure if it'll work with the Nexus 7 since it's a phone plan.
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