I've been on an unlimited Mint plan for five years now. Came from at&t and, in my area, Mint was faster at the time. No idea how they compare today. I wouldn't switch back to another non-MVNO in the states, the experience has been that good.
They keep up with the current tech. 5GUC is in my area since last year, IIRC. They say you get deprioritized vs T-Mobile customers but I honestly haven't seen that. I was kicked off more often on at&t towers than I have been on Mint. That's partly what prompted me to start looking at other options, that and price was ridiculous for what I was getting (6GB/data, unlimited talk/text for $65 - I pay that for fiber at my house!). Dropped the price more than in half and get better, faster, service. Uh yeah!
Only major caveats are that you pay a year in full for the best prices and it's cheapest with BYOD (budget for the device and you can save a boat load, or use Apple's 0% financing for 24 months an borrow their money instead, even better if you have financial discipline). They do offer phones for purchase now though.
I have 1 teenager with his own plan (he works, he pays for it) and he's happy with them too. $15/mo for 3 months and 6GB data but he's on wifi more often than not so that hasn't been an issue. I choose unlimited because I stream all of my music in the car. Streaming video is downgraded to non-HD but I don't care since I'm usually not watching videos when out and about. However, I understand that people pay for HD video streaming since they have a different use case.