I just made the switch from a $30/mo unlimited plan to the $45/mo tethering plan with a limit of 4 gb/mo.
I went to the ATT website and checked my data usage. Mostly about 1/2 gig a month. I am also canceling my ipad data plan now. I even connected my Macbook and my ipad at the same time to the iphone. Even in my small town, I was getting over 2 gbit download speed, running speedtest.net at the same time on both devices.
Win.
P.S. If you go over the 4gb/mo limit, they'll ding you $10/gb. Not a big deal for me as my company pays my cell bill as well as my ipad data plan. I just saved them $15/mo. And, while I was there I checked my text msg usage and dropped my $15 plan to a $10 plan. So now I'm saving them $20/mo.