The main reason to use Verizon is quite simply, to borrow their cliche from their commercials, their network. Verizon is the one carrier that it seems like you can go pretty much anywhere in the US and have your cell connection work. I've gone to more rural areas in the Southeast and the only one that works is Verizon. I live in Florida, and the one carrier that worked during the hurricanes during past years was Verizon. The damn thing just works.
Granted, Verizon is pricey, and GSM is a more worldwide standard, but the bottom line is that Verizon works in the US better than any other carrier. That's why Verizon has the lowest churn rate among US carriers, and is adding customers at a faster pace than any other US carrier. If current trends keep up, Verizon will eventually surpass Cingular for most customers in the US, although Cingular certainly is doing well by their own right.