I've tried. Many times. I really want to love T-Mobile. But I can't. Their customer service is above-average for the industry. Their data speeds are good. Their unlimited plans are really nice to have. Their pricing is amazing. Their international plans are out-of-this-world. But their overall coverage sucks.
I used both voice and data on iPhone 5 two different times - both times in Dallas/Ft. Worth, once in LA, once in Santa Barbara, once in Detroit, and once in Ontario, Canada.
Dallas/Ft. Worth has awesome coverage - I found that I could generally get a strong signal with good data speeds everywhere I went, but that changed quickly as I left the DFW Metroplex.
Service was hardly useable in California - I either couldn't get a signal, or got super slow data speeds, if anything, when I could get a signal, both on 3G and LTE. When I tried a call in Santa Barbara, the voice quality was lacking.
Detroit didn't have signal issues, but didn't have impressive speeds either. I was getting around 1 Mbps on 3G most of the time, and only saw an LTE signal for a split second.
I did enjoy the unlimited roaming in Canada, even at a max of 128 kbps (think airplane wifi). It was nice to iMessage and check email, but it wasn't enough for me to keep service.
AT&T is definitely more expensive, but as momma always said: "You pay for what you get."