If you're wondering if the newly-formed carrier is now bigger than Sprint, the answer is no - T-Mobile US, as the resulting company will be called, is still the fourth-largest operator in the US. T-Mobile USA's respectable 34 million customers have been joined by MetroPCS's 9 million, making for a total of 43 million, while Sprint currently enjoys a customer base of about 55 million.
The plan of the new entity is to gradually merge Metro's with T-Mobile's base, with the whole processes expected to be completed by 2015. After that, MetroPCS's own spectrum will be used for the expansion of T-Mo's LTE network. Clever plan, right? Well, we'll wait and see if it's going to work!