While nearly two million people have brought iPhones onto T-Mobile's network over the years in spite of the fact that the carrier won't actually offer the iPhone until the 12th of April, they've been saddled with service that's not as good as it could have been. Those existing iPhones have come mostly from AT&T and unlocked direct from Apple, and AT&T and T-Mobile are both GSM/HSPA/LTE networks, iPhones programed for AT&T don't utilize the same set of bands as they ideally would on T-Mobile. That's resulted in less than ideal service quality, even if T-Mobile's network is good in the area. According to TmoNews, that's set to change on April 5th, when T-Mobile plans to push a carrier update to iPhones on the network that will bring official visual voicemail and MMS to such devices as well as enabling LTE access for unlocked iPhone 5 users.
T-Mobile's LTE network is still in its early stages, offering 4G service in just a few markets under their new service plans. iPhone users on T-Mobile, be they unlocked or new purchasers of the upcoming T-Mobile iPhone 5, will still have access to Magenta's HSPA+ network. Enabling visual voicemail brings one of the iPhone's early marquee features to T-Mobile users, and while the iPhone has supported MMS for a few years now, that support is dependent on the carrier - and until there was an official Apple-T-Mobile relationship, that carrier support wasn't going to be there.
It's interesting to see that T-Mobile will be issuing a software update to unlocked devices on their network. Those iPhones have come from a mix of sources, primarily customers that have broken free from AT&T and Apple's own unlocked iPhone sales, which Apple started selling in the US two years ago.