Fi currently sells the Nexus 5X, Nexus 6P, Pixel, and Pixel XL. You can also use a Nexus 6 if you already own one.
Pricing for the service is very straight forward. The base cost for a single phone is $20 per month for Unlimited Voice and SMS. Data is charged at $10 per Gigabyte used. You must have at least one gigabyte included in your monthly plan. Tethering is also included at the same rate. The awesome part of the service is that you pay for exactly what you use. If you use less than the amount of data you select, you receive a refund for the exact unused amount. If you go over the amount of data you select, you simply pay for the exact amount of extra data used. No surprises.
You can add additional phones for $15 per phone. The additional phones will pull from the same data bucket, and data is charged at the same rate.
You can order (multiple) data-only SIMs from Fi for free. They simply pull from your existing data bucket. These SIMs are only provisioned for T-Mobile service, and can be used in any device you can shove them into (unlike the full Fi service which requires a Nexus or Pixel phone). There is a separate coverage map for these SIMs, and they do work in some international locations.
Activation, Phone Numbers, and Porting:
At this time, you cannot port in a phone number from a location that T-Mobile does not have access to a number pool. It doesn’t matter if you own the number, or if you have the number in Google Voice. It cannot be done. Hopefully this changes in the future.