As a former VZW employee:
Consider a discount carrier(Sprint/TMobile/Republic/Prepay) if:
1. you live in a city and don't travel much
2. you are not someone who uses a lot of mobile data
3. being permanently connected isn't a primary concern
4. your reason for having a cell phone isn't primarily for safety(see: kids/cars/etc)
5. not worried about cellular connectivity during a storm/emergency
The "second-tier" carriers are great values if you have good coverage at your primary places of existence(home, work).
Go with ATT/Verizon if you:
1. Do not live in a city and/or are traveling outside the burbs often
2. You are someone who values consistent/fast data connectivity
3. Are a techie
4. Permanent connectivity is a business need(sales, technician, self-employed)
5. Primary reason for the phone is safety/security
6. Cellular connectivity during a storm/emergency is a priority
Its pretty simple. If you are in group 1, you are probably going to be buying on price rather than any primary service-based factor. If that is the case, those second-tier carriers are perfect options for you. Sure you will have some dead zones when you get outside the burbs and wont be lighting the world on fire with data speed but its not a big deal. During large storms and emergencies the second-tier carriers often will black out because they do not have the same resources(think tower maintenance, personnel, diesel back-up generators) that the larger carriers have but that's the tradeoff you get for lower costs
If you are in group 2, you value the things ATT/Verizon can provide because of their size/financial strength. You will be a consistent, speedy network in many more locations. You will get greater emergency connectivity. Yes it will cost more. Yes it will be slower to adopt customer-friendly pricing. Yes it will have better customer support. You will get network stability.
Basically, If you rely on your phone for your life or livelihood or as a matter of preference/technology, go with a big boy. If not, save some money and go with the small guys.