There's no downside to T-Mobile. There's no contract, no overage fees, no nonsense. If they have LTE coverage in your city, check them out.
I second this. I also benefited from only paying $40 for using my phone as normal while traveling in France - making calls to the US, making calls to my relatives - for 2 1/2 weeks - and the bill was only $40 more... oh and that's for both me and my wife.
Also I don't have bill anxiety anymore - with AT&T and Verizon, every month I was checking my bill to see if my wife's data quota went over (she refused to pay for 2-3GB, and insisted on the 250MB plan) or if one of my calls went over my monthly quota or if my wife accidentally called her mom's mobile number instead of her home number in France (big rate difference).
Now I pay +$15/mo per line and can call mobile numbers in Europe for free. No worrying about anything. My bill has been constant for the past year and much less than we were paying with either AT&T or Verizon (oh, and we have 3 more lines for relatives on the bill too).
T-mobile has disrupted mobile carriers like Google disrupted webmail in 2004 with GMail.