I think with the T-Mobile One plan, you are forced to use the whole "Binge On" thing. If you're not familiar with it, they throttle your speed so that your phone can only load video at SD instead of HD. I'm not 100% sure on this since I have the old T-Mobile plan which is not unlimited, but offers free unlimited video if I choose to have them throttle the speed at which videos load (which I don't).
I switched to T-Mobile because their marketing team got me. All these videos about how they don't have a coverage issue, just an out-dated image issue seemed convincing. They say their coverage is within 1% of Verizon, but when you do more research you see it's just not true. Sure, their "coverage" is within 1%, but has large gaps between towers with extremely weak signal in between, where Verizon and AT&T are strong throughout this same area of coverage.
I still have the iPhone 6 though which doesn't support band 12, so if you have a 6S or are going to get the 7 then things might be better for you. Also, T-Mobile is rolling out 4x4 MIMO which should improve speeds, but may not necessarily improve coverage. I don't know enough about this to say for certain, so do your research. The iPhone 6S does not support this though and we don't know yet if the iPhone 7 will, but fingers crossed!
My bill is about half of what it was with AT&T, and I have much more data to spend, but I'm not sure it was worth the trade off. I can't use my phone when out and about around the shopping centers or at work. My wife used to talk to me as she was driving home from work, but now it drops the call a few times and it's all broken up the rest of the time.
Sure it's cheaper, but it doesn't come without sacrifice.
Edit: I just checked and the T-Mobile One plan indeed requires you to have your video speed throttled with an option to purchase the ability to override this.
"Customers may choose full-resolution video option (sold separately)."
For an extra $25 per month per line you can have "Unlimited HD video day passes for free", which sounds like unusual wording so I'm not sure exactly why they don't just say it gives you "unlimited HD video"... Bottom line, make sure you read all the fine print and know exactly what you're getting before you sign up. T-Mobile pretends it is all upfront and honest, but IMO they're more convoluted than AT&T and Verizon are.