If you add a tablet to your T-Mobile Simple Choice plan, starting September 3rd T-Mobile will charge you $10-a-month for the privilege and will double your data allotment in the process. The plan creates a duplicate bucket of data reserved for that tablet, up to 5 GB per month. This is in stark contrast to other carriers, where adding a new device, tablet or otherwise, draws from your preexisting bucket of data.
It's worth noting that this is a segregated bucket for the tablet — you've got one bucket for your phone (say, a 3 GB T-Mobile plan) and a separate 3 GB for the tablet.
In addition to the new tablet plans, T-Mobile's also expanding their family plans. Previously limited to five lines on a plan, the Simple Choice family plans now can support a full ten lines. T-Mobile still offers each line with a separated allotment of data, unlike the shared data buckets that its competitors offer.
Just yesterday we saw T-Mobile offer a 2 GB second-tier plan for just $5 over the $40 base plan. They've been on something of a roll since the collapse of the proposed merger with Sprint, though they are facing aggressive pricing competition from the desperate and reinvigorated carrier.