I have been on T-mobile for 10+ years and before was on AT&T. T-mobiles HSPA+ network ("4G") is decent in cities and pathetic in rural areas where it is mainly Edge. I am on their "value" plan where I buy my own phone for a reduce monthly payment and benefit from this over the long term. They also do an interest free installment plan for those can't pay upfront cash.
The US market is the most concentrated one; 80% of the market is between a duopoly of AT&T and Verizon wireless. In such a condition what ever they rule will work. Moreover, Americans do not understand the "value plan" concept and prefer to pay $199 cash down and then high monthly payments. Most of the world, it is buy your phone and then look for a competitive phone company. The 2 year contract system in the US is a way to lock customers for 2 years. Even if you buy your phone AT&T will force you for data plan and contract.
You're right. T-Mobile's biggest challenge is going to be educating consumers on how the Value Plans work. By the responses in this thread you can already see that most people think T-Mobile will be forcing people to pay $649 out the door to get an iPhone when in fact the upfront price to get out the door is likely going to be $99/$199 just like the other carriers.