Not too long ago, unlimited data plans nearly went extinct, but on Thursday T-Mobile launched a new unlimited plan and Sprint cut the cost on another. Before you get too excited, you should know that T-Mobile's offer has more gotchas than a game of hide and seek.

How T-Mobile One works

The new plan is called T-Mobile One, and it offers "unlimited" talk, text, and high-speed 4G LTE data for new and existing customers who want to scrap their old plans. A single T-Mobile One line costs $70, plus the usual fees and taxes. A second line will run you an additional $50, and you can add as many as eight lines on top of them for $20 each.

On the surface, T-Mobile One simplifies things, because it's now the only plan the "Uncarrier" will offer new customers. Not having to worry about blowing through a data cap and getting hit with unexpected charges is a good thing for customers, as well. It will also likely pressure other carriers to make similar moves, or at least offer more data for less money. (Just hours after T-Mobile unveiled T-Mobile One, Sprint cut the price of its unlimited plan from $75 a month to $60. A second line costs another $40 a month, for a total of $100, compared to $120 for two lines on T-Mobile.)

Verizon Wireless no longer offers an unlimited data plan, and AT&T only offers unlimited plans to customers who also subscribe to DirecTV, its satellite television service.

4 gotchas in T-Mobile One 'unlimited' plans

T-Mobile One's terms of service feature a good deal of fine print — and at least four major gotchas.

Let's start with wireless hotspots, a way to share a connection with a laptop or other mobile devices. The $70 plan limits hotspot connectivity to the equivalent of dial-up speed, about 128 Kbps, which makes it useless for all but the simplest of tasks. You simply would not want to use it to tether your laptop. If you want real, 4G hotspot connectivity, you have to pay an extra $15 a month for 5GB of data. And if you go over that allotment, you have to pay another $15.

For some existing customers, T-Mobile One would mean a price increase. Many customers now pay $50 for 2GB of data, for example, so they'd pay an extra $20 a month, though they would also get a lot more data.

As for the whole "unlimited" thing, that claim isn't entirely accurate. Customers who use more than 26GB of data in a billing period will be throttled way down if they connect to congested cell towers.

There's also a video issue. Customers who have been using BingeOn, which provides access to streaming video from many sources without eating into customers' data allotments, are already restricted to 480p video resolution. That's relatively low, but it's also not necessarily noticeable on small smartphone screens. However, it could be more of an issue on tablet. T-Mobile One customers will face the same resolution restriction unless they pay an additional $25 a month for HD video.

T-Mobile One will be available staring September 6, and customers can sign up at T-Mobile stores nationwide and on the company's website (postpaid customers only).