T-Mobile USA finally launched the iPhone today, becoming the last of the U.S. tier-one operators to carry the still-iconic gadget. Users can buy the entry-level iPhone 5 for $99 up front, paying 24 monthly installments of $20 to cover the balance without being locked into a contract. As CNET‘s Maggie Reardon explains, T-Mobile’s new “uncarrier” plans make the iPhone offering pretty compelling — even if its trade-in program isn’t quite as attractive. And T-Mobile’s iPhone is the first to support HD voice, which Gizmodo described as “definitely a noticeable improvement” over typical cell-to-cell audio.

All those factors help explain why we’re seeing some modest lines this morning at T-Mobile retailers. The key to building on that momentum will be in an aggressive marketing campaign, which (as TmoNews reports) kicked off last night in the form of a new TV commercial. There’s a lot to like about the iPhone on T-Mobile compared to the other major carriers, but the new “uncarrier” plans are likely to be confusing at first glance. So the operator must invest heavily in a campaign that explains exactly why its iPhone is a better value than the others’.