https://fi.google.com/about/
This program is only available for users who have the Nexus 6.
 

Project Fi is a program to deliver a fast, easy wireless experience in close partnership with leading carriers, hardware makers, and our users.

 
It has very competitive pricing and if you don't use all of the data on your plan, it'll get credited to next month's bill.
 
You may request an invite if interested; it's only available in the US at the moment.

It appears Google is working on a second generation Nexus 5 which is designed to support Fi.

It could only be a limit in the sense of trying to help Nexus 6 sell; I'd like to think that other segments (especially budget users) will be available in time.

Still, it sort of sounds like Google is emulating Republic Wireless' method of offering unlimited text/voice and data through wifi/cell, since Google's method will seek free wifi in order to save you money.  But Project Fi charges more that RW for that same service.  That said, RW is limited to only Motorola E/G/X models, which are not unlocked and don't accept unlocked models externally.

I can't find the info yet, I wonder if any unlocked Nexus 6 works with Google Project Fi or is it limited to those they sell?

Any Nexus 6 qualifies.

Also, the signal support is also variable, some areas supporting only 2G.