There's a lot more to know.
Basically any potential customer can take a "test drive". You just need a credit card that will withstand a largish ($750?) hold for a week or two. You also need to take very good care of the test drive phone as they will charge you if you don't return it or it comes back broken.
Look not only at T-Mobile's coverage maps but also those on Sensorly.com.
Some people trade in an older phone, keep their current phone, (getting it unlocked), buy a cheap T-Mobile phone but use the phone from their previous carrier. You have to trade in something and buy something to get the ETF paid. Note that T-Mobile has so shaken up the status quo that some carriers (e.g. ATT) are getting away from the subsidy+contract model and going to the EIP model. This means that they have no ETF. If you are buying an ATT phone on an EIP-like plan, there is no service contract and no ETF and nothing for T-Mobile to refund. You need to know your status with your current carrier.
I, personally (IMHO), don't like the trade-in/ETF refund program. Too many moving parts. I recommend you wait 'till the end of your contract, get your phones unlocked and BYOD to T-Mobile.