Ultimately, when it comes to our mobile devices and how we use them, it boils down to signal and coverage. The most powerful processor, brightest, sharpest screen and most proficient camera is worthless when it comes to make and receiving calls, text messages or emails when out and about. Your carrier may offer you a fantastic deal, or free stuff to go alongside your airtime contract but the freebies are wasted if you are unable to connect to the network. Without coverage, the device is as useful as a paperweight for communicating with people.
When it comes to carriers, there is a real strength in having two carriers connected to your device. There are two ways to do this: one is to use a dual SIM device and accept that you may have two bills to pay a month rather one. The other is to use an MVNO, Mobile Virtual Network Operator, that has arrangements with two or more carriers. The advantage of going with an MVNO with connections to multiple networks is that your device will use whatever network has the strongest signal and should seamlessly switch between the operators that your carrier has access too. Depending on the carriers your MVNO is using, this should ensure you have the best coverage, the clearest calls and the highest data speeds wherever you find yourself.
Google's MVNO, Project Fi was just announced and is a multi-carrier operation as the customer gains access to two networks, Sprint and T-Mobile USA. The Project Fi website now has a coverage checker, which you can access by logging on to fi.google.com/coverage and giving the website your location (it will ask you when you sign on). There are two areas of coverage: 4G LTE and 2G. 4G LTE is good news whereas 2G is, to quote the coverage map, "which isn't the best, but you can always text and call over WiFi." If you pick an area of white in the map, the website reports that Project Fi may have coverage in these areas depending on the local coverage offered by their network providers. This seems to reflect that Sprint and T-Mobile US are working on expanding their network offerings.