I've had Verizon in the past, and they screwed me.
I've had PacBell/Cingular/AT&T (one became the other) for about 10 years, and they've treated me VERY well. Of course, I've had no real dealings with 'em lately. Five years ago, I wanted a Motorola RAZR V3i, a model not readily available in the US. Cingular gave me a $200 credit on my bill to go buy myself one, and did NOT extend my contract. A few months later, I got one for about $125, and 'made' $75 on the deal.
Here in Southern California, I suspect all the carriers are good on coverage. I remember when Sprint had dead spots on elevated freeways (way up there in the air?) Different cities have their own unique signature.
I've got a local buddy who thinks the world rises and sets on Windows Phones, and Verizon. Maybe that's his experience.
I've had Sprint for years and I've traveled alot. It I was solely on the Sprint network, yeah, you'll find issues. Fortunately, they have free roaming in the plans. I've been able to get coverage nearly everywhere I go.