Had them about 3 years ago and it was awful. Had to go to one specific room in the house to keep from dropping calls. I would hope they've improved since then.
T-Mobile service is the pits. Their coverage is worse than Sprint, AT&T, and Verizon. Generally it's decent near I-35, but away from the interstate is hit or miss. Parts of Woodway have no reception. Belmead too. Just dumped them a couple months ago for AT&T (5 lines/10GB for $175).