I think what I'll do is pick up a SIM and try it out.
Don't just pick one up in the store if you intend to use the $30 Walmart plan. Word is that T-Mobile is segregating store-bought SIMs. Only those bought on tmobile.com or ones that come with a Walmart phone are eligible for that plan.
T-Mobile store employees are also unable to provision a phone or SIM with that plan. It has to be done on the website or in a Walmart store.
In NYC, T-Mobile is great. Where I live is on EDGE service basically from here to Albany. Along the New York State Thruway, if you glance at your phone, you're either on EDGE or in a dead zone. It's evident that T-Mobile has a long way to go.
$30 is a great deal for that plan, buy only if you spend the vast majority of your time in the right places.