Droid-life just posted a video showing them popping in a brand new unactivated project fi sim and it seems to be working just fine. Of course more testing will have to happen but at this point it looks like it works pretty well.
Posted via Android Central App from a beautiful Ebony backed Original Moto X or the amazing Nexus 10
Based on what I'm seeing on the private Project Fi G+ page, that's not the case. It'll work on one carrier, but won't switch between Sprint, Tmobile, and Wifi. My Nexus 6 does.......usually LOL