Because Project Fi is based on a SIM card, you’ll need a device that has cellular capabilities. You’ll pay a flat fee of $10 per gigabyte of data used, too. That distances Project Fi from more traditional plans, which tend to charge a fee for adding a device to a plan.
Currently, only five devices are supported: the Nexus 7, Nexus 9, Samsung Galaxy Tab S, iPad Air 2 and iPad mini 4. A support page cryptically says various T-Mobile devices may work, but offers no hints as to which ones we can look to.
The lone drawback seems to be tethering; data-only devices can’t be used as a wireless hub on Project Fi.