I don't use it, but it's an MVNO using the Verizon network so anywhere there's native Verizon coverage it should work the same as if you had Verizon also noting that it being an MVNO it probably will not work on networks that Verizon has roaming agreements with and uses the native network only.

I spoke too soon. They were an MVNO of Verizon. From what I understand they are now a Sprint MVNO. Sprint still has good coverage but not as good as Verizon's.