Pam, T-mo is great in the metro area, but had sketchy to non-existent coverage in the Mountains and Wyoming.
The T-mo coverage maps are deceptive! The default setting is to show "projected coverage" for the new Long Range LTE expected to come on-line by the end of 2017. For example, the coverage map shows good coverage around Nederland, but there is none. (You may pick up small spots of coverage.)
I kept T-mobile while I was traveling internationally for business and pleasure for 2-3 months per year.
I left T-mo after 2 years because of the lack of coverage between Estes Park and Black Hawk, poor data coverage along I-70 between Vail and the Utah border (voice coverage is ok), and iffy coverage at ski areas with good coverage on AT&T & Verizon.
I am currently using Cricket 4GB/m $35/m for my phone service since it provides access to the AT&T network in the US. Overall, I have been much happier with the coverage.
On my recent business trip to South Korea, I purchased a SIM near the airport for ~$25 that more than covered my needs for the 5 days I was in country, and I could charge my client for the cost.