A recently surfaced internal document from T-Mobile meant for the eyes of sales managers and possibly floor staff says that the iPhone offers a “poor” customer experience on T-Mobile’s network.
T-Mobile: iPhone offers poor customer experience, 4G Android phones a better option
While it is true that iPhone models aren't compatible with 1700MHz and 2100MHz 3G/4G bands, and can only run on EDGE, according to the latest reports there are approximately 1 million unlocked iPhones on the pink carrier's network.
The internal document goes on to advise sales reps on how to educate customers and get them to opt for 4G HSPA+ Android phones instead
Of course, as TMoNews states, one could argue that while the data speeds may be limited, the overall experience won't necessarily be considered poor given that the hardware and software is better than many other T-Mobile 4G phones available at the moment.