I've always been pretty happy with ATT. It annoys me sometimes when I lose signal in the middle of fricken silicon valley, but it's not a constant problem.
On a recent road trip into the mountains, I did notice that my buddy's Verizon phone had signal everywhere, long after my ATT signal was gone. It was frustrating. So, when the iPhone 5 comes out, I'm going to switch to Verizon and give them a try.