Carriers that didn't pay the dreaded iPhone tax are feeling the pinch and customers are leaving in droves. Smaller players such as MetroPCS have been hanging on by the fingernails since iPhone 4.
Leap (Cricket) has had the iPhone for a little wile now and they're still struggling.
Well you have to factor in the price and what they offer. If they give you over 2.5GB of data under 60 then you will see those telcos doing better. Look at what happen to H2O, they started great and then they went to the 1GB max and people left in droves to Straight talk.