T-Mobile US has been everybody's darling for a while now. They have lowered rates, improved their network and got a lot of new customers in the US.
But, step by step, they have introduced small price hikes since the beginning of the year, which hurt short-time visitors especially.
1.) The official sales price of a T-Mobile starter pack called activation kit in branded stores has been raised from $10 to $15 plus taxes. This is quite high as you are given a only minimal balance. Better buy it from Ebay or Amazon merchants who get it in promotions for free and resell it cheaper. But remember: You can't activate it on a foreign roaming network, you need to be in the US on T-Mobile.
2.) The have introduced a new "pay as you go plan" for $3 per month. This includes 30 mins of voice and 30 domestic texts. This is the only plan on prepaid which is not on a monthly base. So, in effect, you have a $3 minimum charge, even if you are not using their service, because you are out of the country and only want to keep you SIM alive.
3.) They discontinued all "pay by day" plans by February 2015, which used to be a good option for short-time visitors. Previous customers on these plans have grandfathering, but new customers are not allowed on the daily plans anymore.
So for travelers MVNOs and AT&T may again be good alternatives. I think this website gives a very good, and updated survey:
http://www.prepaidphonenews.com/2011...-and-data.html