T-Mobile just announced a surprise Uncarrier program that will do away with all its current plans. Instead, there's only one plan that T-Mobile is calling T-Mobile One. It has unlimited data, which is great. However, it's not really an unlimited plan. There are plenty of limits that you should be aware of.
The way John Legere leads with the pricing below is a little confusing. He clarifies later, but the gist is that it's not actually $40 per line overall—that's just the average if you have four lines. The actual pricing for T-Mobile One is $70 for the first line, $50 for the second, and $20 for lines 3-8. These are pretty solid prices, especially for unlimited data. T-Mobile is presenting this as a single plan, but it's actually a single plan with a bunch of add-ons you need to buy in order to get the same functionality you have now.
The "unlimited" data on T-Mobile One basically has Binge On activated all the time. If you want HD video, you need to pay a whopping $25 per line. Tethering is technically unlimited on One, but it's throttled to 2G speeds (thus, it is useless). If you want LTE tethering, it's limited to 5GB and costs you an additional $15. Also, the plan prices are actually $5 higher by default. You have to add autopay to get the tiers as listed above.
So, here's an example: two lines on One cost $120 with unlimited data (and autopay). Great. If you want tethering and HD video, which are included in the old plans, the cost jumps to $200. That's a much less appealing value. The new plans are live on September 6th for postpaid and at some point later for prepaid.