The first four officially supported tablets are the iPad Air 2, iPad mini 4, Nexus 7, Nexus 9 and Galaxy Tab S.
Project Fi customers can order a data-only SIM card through their account page: I checked this morning but the card isn't yet available for my account. There's no charge for the SIM card and data usage is billed at Project Fi's standard rate, which is $10 per GB of usage during a billing cycle.
Data usage for tablets shares the same billing account as a standard voice, text and data plan under Project Fi, although you can see how much data your tablet has used in month separately from the data on your phone. And like the phone service, if you don't use your full data allotment on your tablet, you won't pay for it. Google credits Project Fi accounts monthly so you're only paying for what you actually use.
I wouldn't suggest that data at $10 a month is the best deal available, of course. Our AT&T family plan includes 15 GB of shared LTE data, for example and even though adding a tablet to that plan costs $10, I can use more than 1 GB per month out of the shared poll.
Still, some Project Fi users may appreciate the data-only SIM card offering. They could certainly use their handset with a Project Fi SIM card to tether a tablet over Wi-Fi but if their phone battery dies, their tablet has no mobile broadband connection. Road warriors may appreciate the new SIM card as well since it provides data in more than 120 countries around the world.
There are some small limitations with Project Fi data service, however. Google only allows 9 data-specific SIM cards per Fi account; not likely an issue since we're not yet carrying 10 devices with us. (Frankly, I hope that never happens.)
And setup isn't quite seamless. Regardless of whether you have an Android or iOS tablet, you'll have to tweak the device's APN settings so that the SIM card and Fi network is recognizable.