Three years ago on Sunday, Google launched Project Fi. The MVNO (Mobile Virtual Network Operator) solution combined T-Mobile and Sprint as carrier options for the Nexus 6 and allowed for seamless switching between the two carriers automatically. Fast forward to today, you still have T-Mobile and Sprint in the mix but now you also have U.S. Cellular in there for additional coverage.
The idea was a unique one and remains so today. Project Fi also brought low cost data usage at the time of its launch, particularly for those who traveled internationally. That part of the service has improved too as you are not longer as severely throttled on your data speed as you were back three years ago. As for the cost, it still remains mighty competitive in the market.