T-Mobile's unlimited talk, text, and data plan is the big carrier option to beat at $80 a month. But so far it's only been available to individual customers - if you have four people and you want four lines of unlimited data, you need four different accounts. Starting tomorrow, December 10th, unlimited data will be available for families as well. The cheapest option is unlimited talk, text, and data on two lines for $100 a month, a $60 savings over the old structure.
Expanding your cell phone family is similarly thrifty, at least in the realm of big bucket data. Each additional line with unlimited everything costs an extra $40, up to ten separate lines. It's an impressive alternative to the current promotion, which uses 10GB of shared data for up to 10 lines for a flat $100. The only limitation is that each phone needs to be an LTE model, which shouldn't be difficult to obtain.
T-Mobile's plans don't include subsidies, so if you intend to switch to the carrier, you'll need a T-Mo LTE compatible phone (like the Nexus 5 or 6) or you'll have to either buy one outright or sign up for an EDGE installment plan. Even so, those who go through huge amounts of data can still probably save a ton of cash versus AT&T or Verizon's metered plans. It's just a shame that you can't sign up for this plan and take it over to Sprint's half-off promotion... they're not accepting T-Mobile bills.