So far, what I have gathered from the Google Project Fi experiment is that they've turned Google Voice into an MVNO and re-branded it as Project Fi. It integrates with your Hangouts account for messaging and calls and then furthers that experience by piggy backing off two mobile networks for when you aren't around.
It's a great idea in theory but I believe they're approaching the selling points all wrong -- they're saying it's a great way to save on data, but the problem with that is my T-Mobile line is actually cheaper. I use 15Gb per month, give or take, therefore I'd be paying $170 for their service. They're pricing their data comparable to AT&T or Verizon but utilizing the lesser networks of Sprint and T-Mobile.
I say lesser due to coverage, my T-Mobile has far outperformed my former AT&T in the Los Angeles area. I understand they have an emphasis on wifi but the common consumer isn't really thinking oh I have to turn on wifi to save money. Most people I know don't even have wifi enabled on their phones because their service is strong enough or stronger than most wifi connections.
The connected cloud idea is great, given that many have Google services therefore always have some form of Hangouts available to them. But it's really no different than iMessage, a current Google Voice number, or what WhatsApp has done on Android allowing you to use a web interface.
I think it's a great concept, but I feel it will have a very slim user-base, similar to their nexus devices. It isn't something that's very consumer oriented, regardless of how it's sold -- requirement of a nexus 6 is proof of this theory. I don't know many consumers that carry a nexus device, let alone one that's 11" tall.
I believe they will need to work toward becoming a physical wireless provider the same way they have done with Google Fiber, because without the granular control they have at the base level, they will never be able to do anything in regards to pricing while utilizing other's networks.
I'm interested in seeing where it will lead, I'm always fond of many of the projects that Google brings to fruition -- I just wish the utility was there for me to try it, but it isn't worth the extra $100 per month to do so.