We use Millenicom for internet . They sell service from Verizon and Sprint. Our plan is 20G through Verizon for $60.00/mo. No contracts. I am not sure what plans they are offering now. Someone said they change from time to time depending on the air cards they have at the moment. We did have to buy a $99.00 Verizon air card.

We checked out jackdanmayer.com site. He has the best information on this.

Then, after reading Jack 's information we got a WiFi Ranger router. We are all set up. It was easy and we are very pleased.
We monitor our usage through the Verizon web site (the air card has a phone number), but we have only used 8G one month.
We have more than enough.

I can second the choice of Millenicom; 20 GB on their Advanced Plan for 60 a month. The Advanced Plan runs on Verizon but they're currently not offering it due to a shortage of 3G cards. You do want to put an antenna on the roof for the 3G card; that will increase the signal considerably. We also have the WiFi boost for our WiFi Ranger (this is an external antenna which pulls in a lot more wifi signals). 

I hope you do go to Jack Mayer's site, http://jackdanmayer.com/communication.htm#Internet on the Road , to see what he says.

Now a couple questions:
1) Do you have any idea of how much usage you do in a month? If you are running Windows you can install "networx" (do a search) for free on each machine to see your real usage. It is important to know your usage since most plans care.
2) Are you in one place all the time or moving? I ask this because you asked about AT&T, which traditionally has not had the best internet service for mobile users. For phones AT&T and Verizon are neck and neck for voice coverage, but data (internet) coverage the main choices are Verizon with Sprint second and AT&T a distant third. But AT&T is now moving into a race with Verizon on 4G/LTE data coverage and could be a solid mobile choice in a couple years. AT&T needs a 2-year commitment for 5GB per month using a hotspot or USB modem. If you are staying in one place, AT&T is great there and 5GB is plenty, it can be a good choice.

Jack's site is awesome.. and gives a lot of great info.

We also offer up our tips for keeping connected here:
http://www.technomadia.com/2011/09/10-tips-to-keep-connected-us-mobile-internet-options/

We utilize Millenicom as well, on their old Advanced Plan for 20GB/mo at $59.99 on Verizon.  And we use AT&T tethering via an iPhone as our redundancy for cellular (by the way, despite what is said above - AT&T's network has worked great for us in our full time travels all over the place, and their 3G is much faster in many places than Verizon's 3G because they use a different technology... and we've had plenty of places we've needed AT&T because Verizon was absent).  

We also have a WiFi Ranger Pro & Air setup for bringing in WiFi more reliability, and a Top Signal 55 cellular booster to help with the cell signal.  If all those fail, we carry a satellite tripod setup that we can activate as needed. 

We rely on the internet for our livelyhood, so we try to keep all our bases covered. 

Best wishes,

 - Cherie

I used to have an AT&T phone that showed "G" for old 2G, "E" for Edge (2.5G) and "3G" for 3G. I found many areas in places like the Dakotas I only found "G" and/or "E", which is slow. Remember when Verizon got the iPhone and many were excited since they could finally get one since AT&T had no service where they were? As always it depends on where you travel, Florida is a great state for AT&T, New Mexico, Montana, the Dakotas, Oregon, not so much. Verizon has had slow service, which they call 1X, in places also like many areas of Maine and the Washington State Coast around Long Beach (which we care a lot about since we both have family there and love the area). But it seems that if you can have both AT&T and Verizon you cover most areas that have cell phone service in the United States, for both voice and data. Leave the U.S., you better have satellite internet. We were really glad to have satellite internet and to be on the Horizons-1 satellite on our trip to Alaska.

I have no direct AT&T info since June 2011, when we went to a Verizon family plan.