Had Verizon (Alltell before them). On ATT now, over 2 years. Travel (camp) all over the State primarily north of US-10.
ATT coverage is as good as Verizon for voice. Each has weak spots in select areas. Some of the remote campgrounds we hit, ATT will be so-so and VZ will be just that tiny bit better; others are the opposite, most are the same.
ATT is generally EDGE data (the slow speed) for 99%, except some major areas. It will be better by/in 2014 as they are dropping EDGE spectrum to light up 4G; whether that means it will be 3G where it's currently EDGE, don't know.
I plan to switch back to VZ shortly as I believe they have better data speeds in northern MI. That's based on anecdotal evidence from folks with VZ phones that I check with at camp (and not just ask, have them show/use their phone). If we got 3G everywhere we get EDGE now, I'd stick with ATT.
Camped near Honor this weekend and had 3G. Easily able to full screen NetFlix while tethering off the phone.