T-Mobile finally gets an iPhone of its own this Friday, letting Apple fans enjoy the full speeds of the carrier’s network for the first time ever. As such, T-Mobile is really invested in making this launch a big success, and that means showing the other carriers that, even late to the table, it can still sell a whole mess of iPhones. To help draw customers, it’s offering a trade-in deal for previous iPhone owners, but is this promotion really a good deal? Normally,