AT&T isn't so great where I live and work. Lots of dropped calls on my iPhone 4 and my wife's 3GS. Data ping times often hit 2000ms (yes, two seconds) when I run the speed check apps and sometimes data fails completely despite having 4-5 bars of signal. And there is near zero AT&T signal penetration into the building in which I work.
Meanwhile, my Verizon phone hasn't dropped a call in 6 months and I get total coverage inside the building at work.
If AT&T works well for you, then there isn't a need to switch. But for me, I couldn't be more excited.