I've been long-term customer of all major carriers. They all suck in important ways, but T-Mobile seems to be working sincerely to be positively disruptive in support of customers...and a leader among their peers in terms of "corporate social responsibility"
Switched from AT&T and it is much cheaper for us as a family (who use a lot of data). Best carrier for traveling out of the country with free (but slow) roaming. Nice not to have to worry about large surcharges. Biggest downside is their cell coverage isn't as good as AT&T, but not bad enough to overcome the positives.
With heavy data consuming kids and some recent trips outside the US, T-Mo's unlimited data plan including international data has been worth its weight in gold. Service coverage is a step down from AT&T, but paying nearly $80 / month less for family of 5 is a no brainer.
When they pioneered free data in Canada (even at limited rates), I became a fan. When they doubled down and made it 4G free service, I became a loyal fan.