It took T-Mobile long enough to get the iPhone. All of the other major carriers, and even some minor carriers, have had it for a good period of time. We knew that T-Mobile would eventually get the iPhone as they said that they would be getting it this year. At the time, we weren't sure when it would be arriving.
For users looking to pick up the iPhone 5, you'll have to put $99 down and pay $20 per month for the next two years. This works out to a cheaper upfront cost, though you do have to pay the full price of the device over its life.
The iPhone 4S will set you back $70 at the outset and the same $20 per month. If you're into really old tech, the iPhone 4 can be had for just $15 down and $15 per month.