Last Updated: 05 October 2016
Project Fi is a new kind of phone service....
The service preferentially uses wi-fi connections for all services, and if wi-fi is not available, can automatically select and switch between T-Mobile and Sprint mobile LTE services. Data is $10 per Gigabyte; and you pay only for what you use.
International 3-G and LTE data (140 countries) is also just $10/GB. Free text messaging is included. Voice calling over wireless, 3G and LTE networks is relatively inexpensive, $0.20 or less to/from most of the 120 supported countries.
I just started with the service on July 30, 2015. So far, I've used the service in the US, Republic of Ireland, Northern Ireland, Scotland, England, Portugal and Spain. It works as advertised, with a few minor glitches in international use. When entering a new country or "service area" and attempting a voice call, the phone would often state that it wasn't provisioned. Turning the phone off and back on cures that. Total international "roaming" cost for a two week period was less than $2 even with frequent use of Messenger, GMail, News & Weather, Google Maps, Google+, and Facebook. On my latest trip to Portugal and Spain, LTE data speeds were better than hotel wireless in many areas.
One nice feature is automatic voice-to-text "transcription" of voice messages. Does a great job, very few mistaken words, and every call-back number left in a voice mail so fare has been correct.
The service currently requires a Nexus 6 phone, which is larger than most phones. Beautiful high-res screen, very fast, and good battery life. The display is a bit hard to see in very bright sunlight. The phone does fit in most men's trouser pockets, and (more importantly) in my motorcycle jacket pockets. It does not fit in most women's pockets, however.
Update: Unexpected Google Fi holiday gift arrived on 28 Nov 2015 - a Lego set that can be used to build multiple Nexus 6 accessories like a stand, or USB cable organizer.
Update: In eary 2016, the service was opened to all applicants (vice by invitation only). In July 2016, Google announced an agreement with service provider Three that provides tens times faster international data.