All things considered, Project Fi is certainly worth a look, particularly if your home or office has crummy cellular connections. And it's certainly cheap.
Project Fi is Nexus-only ... for now
So why does Google make you use a Nexus phone? According to the Project Fi FAQ page, the radios in those phones support multiple networks, unlike many other popular phones that do not. Google hasn't said whether or not it plans to support other phones in the future.
If you already own one of the compatible Nexus phones, it will work on Project Fi. If you sign up by April 7, you can buy a discounted Nexus 5X from Google for $199, or $150 less than its regular price. Project Fi doesn't require any contracts, and Nexus phones purchased through Google are unlocked, so you can use them on different carriers' networks.
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