Google's invite-only Project Fi MVNO experiment is still small, but today Google is expanding it beyond phones for a small subset of devices. Existing Project Fi subscribers (so, people who have received an invite and are using a Nexus 6, 5X, or 6P) will soon be able to add tablets to their plans using new data-only Project Fi SIM cards.
You can connect up to nine data-only Project Fi devices to your account, and they all use the same pool of data as your phone. If you use 500MB of data on your phone and 250MB of data on two tablets, you'll still only be charged for 1GB of data. Most crucially, Google doesn't charge users "device fees" to connect their tablets to Project Fi in the first place. Most of the major US carriers will let you connect a tablet to the same pool of data as your phone, but these companies will charge you a $10-ish flat rate per month regardless of how much data you actually use. You'll still need to pay the base $20-per-month for Project Fi service, regardless of how many devices you connect to your account.
The data-only Project Fi SIMs only work on T-Mobile's network, not Sprint's, and they officially support five Android and iOS tablets so far: the 2013 Nexus 7, the Nexus 9, Samsung's Galaxy Tab S, and the iPad Air 2 and Mini 4. The iPad and its app ecosystem are generally stronger than what you can get from Android tablets, so it's nice to be able to mix-and-match if you like Android phones but prefer iOS tablets.