Of course coverage varies greatly from area to area, so that leaves customer service, billing.
Compared to AT&T wireless, it's great.
Typical call to AT&T wireless took 30-45 minutes & I had to speak with 2-4 different people, with varying attitudes, can't tell you how many times "Twanda & Jill" have accidentally cut me off during a CS call.
Typical call to T-mobile, 5-15 minutes, speak with at most, 2 people, and they're friendly.
Phone replacement is prompt & painless.
Billing, AT&T screwed up my bill every 2-3 months, and it took the typical 30-45 minutes to fix it, the error was always in their favor.
T-mobile 2 billing errors, both fixed within 15 minutes, last one was after talking to them, a legit bill, but I talked them into eating a $20 charge on a number I had rolled over into a family plan.
Phone replacement was brutal, I sent a broken phone to AT&T's CEO because they would not provide me with an address to send my broken phone. After a month & numerous pleading calls, I googled the guy's name & sent it to him certified, return reciept requested, at their corporate HQ.