I looked into this because who hates free cell service, and Sprint is decent in my area. Unfortunately, I'm with Google Fi right now, and since Google Fi is basically a pre-paid MVNO of Sprint, my phone and number are not eligible. Apparently, getting a Nexus 5X bought through Google Fi on to Sprint at all is a bit of a challenge.
I will have to continue to suffer with blazing fast 4G all around the world for my wife and me for around $45/month total, instead. #MustachianProblems. Oh, and my past experience a loooooong time ago with Sprint customer service did not go well. Google Fi easily has the best customer service (though not >$40/month better.)