We have been roaming the SW US lately and have added Straight Talk (ST) to our phones. There are several options among ST services. We chose the Bring Your Own Phone (BYOP) plan. Our trips have been from Denver to Dallas, Salt Lake City, Grand Canyon and Kansas City.
Generally, I can say that ST stays with your "Home" network and will not roam as long as you are within that network's service area. If, for example, your ST/AT&T phone gets no coverage in an urban area, it will not seek a roaming partner...they appear to be geographically restricted. When you are out of that carrier's coverage area, the normal TracFone roaming agreements apply. More specifically, when I lose coverage, like in rural Colorado east of Denver, my ST/AT&T SIM switches to Commnet Wireless and not to the usable T-Mobile signal.
In other locations, like the I-25-Colorado/New Mexico border and in Utah outside of Moab, the phone immediately picks up AT&T. In other areas, an ST phone might switch more readily to the smaller local carrier, but will eventually return to the "Home" network as soon as the phone finds it. In these roaming areas data is Edge speed or higher. Most of these roaming variables are affected by the timing of inquiries by the phone.
The black hole on our trips is when GSM coverage is provided by Verizon on the ex-Alltel GSM sites, which will eventually go away. Hopefully, someone will fill these with new GSM coverage. It's the only places where I found no data available. The longest stretch is US 64 from Raton, NM to Dalhart, TX.
In northwest Texas off the Interstates, all GSM phones jump around quite a bit, roaming on AT&T more than others, but I will assume other roaming partners, include Verizon/ex-Alltel which provides a voice/text-only service. Data was really variable. The good news is that the "Home" networks are more built out each year.
On average, the ST phones give us coverage in all places. If you were to switch from a Verizon phone to a Straight Talk phone, you would still get your calls. If you were to choose one of their single-network phones, the story is much different. Even with a ST/Verizon phone, the idea of being limited to one network is a concern.
We have added a complete report on all of the America Movil networks which includes Straight Talk, TracFone, Net10, SIMple Mobile and the new Telcel America.