I got rid of my Galaxy S4 and hoped on the OnePlus train about a year ago. Since the OnePlus was GSM only phone I switched from Verizon to T-Mobile. I live in the Philadelphia metro area and almost always had a good LTE connection. I was on there $80 unlimited everything plan and used about 20gbs a month. These is my thoughts after a year of use.
Speed: I averaged 20 to 30mbps on LTE. (60 to 80mbps next to a tower). I was never throttled even with my high data usage.
Reliability: LTE data went out one day nationwide. No problems other then that.
Call Quality: I rarely make calls but the ones I did sounded just fine.
Coverage: In metro areas you will have LTE. You will struggle in large buildings to get LTE since T-Mobile uses higher frequency bands that don't penetrate walls very well. Along highways you will have on and off LTE or solid 2G. In rural areas its solid 2G or roaming (Roaming is included free in most plans).
Pricing: $80 for unlimited everything is a great deal for data hogs like me. Other plans competitive with Verizon and AT&T.
Extras: Unlimited plans get free rhapsody music streaming which I use a bit.
Conclusion: T-Mobile has made tremendous progress in the last year but still have a long way until they beat AT&T or Verizon. If you are a data hog or live in a metro area then T-Mobile would be a good choice. If you live in a more rural area or use a only a little bit of data each month then I would look somewhere else.
We switched from AT&T to T-Mobile about a year ago and it was the best thing we ever did. I had verizon before, but they're pricing for what you get is absurd. And when we moved locations prior to AT&T Verizon had the worst service.
The best thing for us with T-Mobile has been the Wifi calling, it has been godsend in our area, as all the providers have sketchy signal around the neighborhood.
What do you use so much mobile data for? I use 700MB on average each month (of which over half is usually snapchat or youtube). No disrespect intended; just curious!
I've been on T-Mobile for the past 4 years now and I have to say that I like it a lot. I live in NC so some of the more rural areas may have spotty coverage every now and then but other than that it's been perfect. I use a lot of data each month; Spotify, YouTube, Plex, Google Hangouts, etc. and I've never had an issue with speed or throttling, granted I'm on the unlimited everything plan as well.
Last weekend my internet was out all Saturday and Sunday and I was able to use tethering for everything from downloading some light games on Steam to a few Skype calls and Teamspeak usage. My speeds aren't quite as fast as what you're getting but they're still fantastic. I mean, I get a faster upload speed on my phone than I do at home with TWC. ;)
The only real issue I can find with my T-Mobile service is coverage in some areas. I don't travel all that often, but when I do service can be a little hit or miss sometimes. That being said I was in the middle of nowhere in Colorado a few years ago and I had perfect coverage and LTE which was pretty astonishing.
I realize I'm rambling here but those are just my thoughts. :)