The skinny of the story is that they simply used a Nexus 6 to activate Project Fi service, and made use of a nanosim-to-microsim adapter to use the SIM card inside of a Nexus 5. Service was said to be perfectly functional for incoming and outgoing calls, as well as full LTE data, but the user was unable to verify whether Google’s hand-off technology (to automatically switch between Sprint, T-Mobile and WiFi) was working.
There’s also the issue that there haven’t been many other reports of success. Most people who have tried (and failed) to activate Fi for use on a Nexus 5 tried to activate service on the Nexus 5 itself, but to our knowledge this is the first and only report of someone using a Nexus 6 to give the initial activation a go.
There are a couple of issues to note for anyone whose ears suddenly perked up at the thought of not having to own a Nexus 6 to use Project Fi:
It’s unlikely Google’s network hand-off technology will be supported, as Google originally confirmed there’s a mixture of software and hardware components needed to facilitate it.
Your device could be limited to use on T-Mobile’s network only as Sprint’s network requires an MEID number that’s been approved for service.
So, all of that is to say that using Project Fi on anything other than a Nexus 6 would be akin to using a smartphone on any standard MVNO carrier. Unless you’re a huge fan of how Google runs the administrative side of the service — such as pricing (which isn’t even the best value you can find) and plan features — then there’s really no real current benefit in using Fi on other devices.
But if all of that is OK with you then, by all means, be sure to give it a go if you can borrow a friend’s Nexus 6 for activation. Information is still being gathered so there are still a lot of questions as to the specifics of how Fi service would work on different devices. If you happen to try this approach be sure to report back with your results — whether that’s here or over at the original XDA thread — to help contribute whatever information you can.