I changed to T-Mobile from AT&T a few months ago. Cell service in and around my home is average (2 of 4 bars, slightly less than I had with AT&T). In quite a few cases, when talking on our cell phones at home, the people we are talking to say they cannot hear us or we sound far away. We did not experience this with AT&T.
We frequently find when we are inside buildings in our home are that we have not cell service. The only time I encountered this with AT&T was inside hospitals.
We recently traveled to Pittsburgh. We were in a suburb of Pittsburgh (12 miles south of the city) and found we had no cell service at all. We used to live in this same area and never experienced no cell service (we had Verizon and AT&T while we were living there). During our drive between SC and PA, we encountered many, many areas with no cell service (some rural and some urban). The best cell service we encountered was in WV (not everywhere, but the majority of the route), but to our surprise, the service was provided by AT&T.
In NC, our phones indicated 2 bars of cell service. We were able to make calls, but the calls kept dropping repeatedly.
We are using the same phones (iPhone 6) that we used with AT&T, so the differences and issues we are encountering with T-Mobile would appear to be related to the quality of T-Mobile cell service.
We are traveling to Italy later this year and were initially excited that our plan includes talk, text, Internet while we are there. But we are now wondering if we will actually have cell service to get those services.
We switched to T-Mobile because of the features and plans/pricing, but the experience we have with the actual cell service has been disappointing.