I had been on Project Fi, and my phone before Fi was a Nexus 5. I am also a serious Google Voice user. There were a few problems I have encountered with real world use. First, is the limited phones. Yes, you can (as of Jan 12, 2016) can get a reasonable priced Nexus 6, but for the most part, most people will have to pay a small chunk for a new phone. This is a deterrent if you are happy with your current phone that may not be one of the 3 only options. Second was call control. Everyone had my Google Voice number, and when I was with Project Fi, everyone called my cellular which is fine except when I didn’t want it. I used call groups to direct less important calls to my SIP client which only rang when I wasn’t home. Also, no SPAM controls. I had to create a “contact” and set that contact to ring to voicemail. However, this still rang a Hangouts device. Couldn’t do that with Fi. It was all or nothing. Google controls both Fi, and Voice – you would figure that they would have given the best of both, but they missed the mark a great deal. I do like the value (although data cost is a little high), and customer service was great (when they could have helped). I am not saying Fi is bad, just bad if you relied on Google Voice.