I have been using T-Mobile as my carrier (not necessarily 'of choice') because I'm on a family plan with my mom who is way up north in MN. It was about the only way we could afford a cell phone for both my wife and myself (we pay 1/2 the bill).
Would I have picked T-Mobile otherwise? No, because they aren't local to where I am. And, knowing what I know now, their reception is marginal in most places I go to here in the Iowa City area and I'm not that impressed with their customer service plus a few other quirks that rub me the wrong way. It honestly baffles me the number of rave reviews I see for T-Mobile.
When our current contract is up, I'm going to bug my mom to see about a family plan with Verizon, who my dad has his phone with. Or maybe I will see what my brother is using and ride on his coat-tails.