Before you get too excited: you have to request an invite to sign up for Project Fi, and it's only available for the Nexus 6 now. Bummer. But still, if Google opens up the project to other phones and devices, it might be worth switching.
The pricing is pretty straightforward: $20 a month for unlimited talk and text (including international texts) and Wi-Fi tethering. Coverage is in over 120 countries. Data costs $10 per GB (so $10 for 1GB, $20 for 2GB, and so on). The best part, however, is you don't pay for unused data. It's not a rollover plan where you can use the data next month; instead you get a credit back, so it's truly pay as you go.
Another benefit is Project Fi runs on two carriers, Sprint and T-Mobile, which could increase your coverage area. It also uses Wi-Fi when available.
I ran the numbers, comparing my T-Mobile plan to Project Fi, and Project Fi is about $10 more than my family plan (four devices, one with unlimited data at $20 a month, two with 3GB of data at $10 per month, and one with no data). T-Mobile's Data Stash rolls over unused data into the next month.
Google's prices for data seem a bit steep, but I appreciate that you get a credit back for unused data, and the inclusion of tethering without an additional cost.
For heavy data users, it might not be the best deal, but for those whose data needs vary each month or are low, it might be worth a look. If you have a Nexus 6 or when Project Fi opens up to other devices, that is.