Re: current AT&T users. happy? not happy?
I had a smartphone on ATT since the original iPhone until last year. Their service was always average where I lived in Colorado Springs with a few well known dead zones. I traveled to the Long Island/NYC area frequently as well as Wisconsin/Chicago from time to time and service in Wisconsin was terrible, almost entirely edge roaming and wasn't reliable at all. Same went for NY except it was their native network and was also incredibly unreliable for me. I moved to Boulder, CO a few years ago and had almost no reception in my house and ATT said they couldn't do anything about it, I switched to Verizon and have never looked back after being fed up with ATT. I have full LTE coverage everywhere in Boulder, Colorado Springs, Illinois and the NYC/Long Island area with reliable data and even more reliable calls. In Wisconsin I at least get reliable 3g roaming now which is better than nothing. I have maybe dropped less than 10 calls on Verizon if that over the last year where ATT I couldn't even count how many but I could predict when it would happen since it happened almost like clockwork.
Obviously my experience won't be the same for everyone, but I travel all over the country and can say that Verizon is much more consistent in the vast majority of locations versus ATT in my personal experience. I'm not a Verizon fanboy or an ATT hater, I would absolutely love to go back to ATT if they got their coverage in order; I've had similar customer service experiences on both as well, maybe a bit better on Verizon so that's never been an issue for me. The reason I would love to go back to ATT is because I much prefer the GSM standard over CDMA+LTE for many reasons, but not enough to sacrifice on coverage, because ATT simply is very bad in the locations I most frequent in my personal experience.
I recommend trying it out before you commit, if the places you spend the most time in have good ATT coverage I would recommend them, since when I had good ATT coverage their call quality and data speeds were excellent, but my problem was the bad spots out numbered the good spots too often.
I have a prepaid ATT sim card that I throw in my Verizon iPhone 5 to test them out for a bit to see if they have improved anything, The signal in my house has seemed to improve a bit, but my data speeds are still less than 1mbps on HSPA+ and no LTE in my area still. When on Verizon LTE I get 12+ on average and on EVDO I get 1.5-2 mbps, which is sad that EVDO beats HSPA+ in my house.