T-Mobile doesn’t yet sell the iPhone 5, with T-Mobile only offering 2G data-speeds in the United States. T-Mobile though is working on fixing this, making it a great network to go onto with an unlocked phone.
T-Mobile does offer you much affordable plans than what you get at AT&T and Verizon and if you have the patience to wait, you will be onto their faster HSPA+ speeds very soon. Some areas already do have these speeds, so do be sure to check that out.
The unlocked iPhone 5 also supports the 1700MHz/2100MHz frequencies which will be what T-Mobile will be using for their LTE network, coming next year.