Google's Project Fi, a cellular service allows you to use Sprint and T-Mobile's networks together on a single device, is now offering data-only SIM cards for use with tablets and other devices. You can order a data-only SIM if you have already activated Fi service, and tablets with these cards will have the same data price as your phone, $10 per gigabyte. When checking your data usage, it will be broken down by device once you install a data-only SIM.
In addition to the three Android tablets that google knows to be compatible with Project Fi, iPad Air 2 and iPad mini 4 owners can also take advantage of data-only SIM cards. This is the first time that Apple devices have been officially compatible with the project.
Project Fi's support page says that the data-only SIM could work in other devices (just make sure they're T-Mobile compatible), but beyond the above list the carrier won't be inclined to support or help you with issues if you choose to use a non-approved device. The page even goes so far as to say you could use the data-only SIM in a second phone, if you wish to just have data and not calls or texts.
As for other limitations, you can add up to nine data-only SIM cards to your account, though you can use the same card in multiple devices. Additionally, you cannot tether from a device with a data-only SIM. The ability to order these cards will be rolling out gradually to existing Fi users, and the option should appear in the next couple of days.