I'm halfway through my first month with Google's Project Fi on my Nexus 6, Fi is pretty neat.. I've definitely been taking advantage of the use of WiFi and unlimited texting/calling.. I have unlimited texting on my Verizon line but not voice minutes. Anywho..
After a couple weeks with the service, a few little trips.. I can say it's definitely not ready for primetime yet, but it does show promise. The pricing is a little skewed for me, if it was my sole cellular phone provision, I could see the price structure, I'm on the 1Gb plan, meaning no more Spotify streaming in the car, I cache everything offline then stream from playlists locally on the phone. I go out of my way to utilize WiFi when and wherever I can, like my trip to King's Island last night.. park-wide WiFi so I didn't have to worry about data consumption. T-Mobile & Sprint are probably really great for some people in some areas. I know part of the problem with T-Mobile specifically in my area is that 700Mhz Band 12 spectrum is blocked by digital channel 51 out of Louisville, so I can't access it. I do regularly have T-Mobile LTE though, there is nothing here locally between Edge and LTE with them though. I've seen Sprint pop-up a few times locally whenever a building keeps T-Mobile out, which without the 700Mhz spectrum is pretty easy to do. Whenever I'm on the Sprint side of things, it isn't pretty I had Sprint LTE yesterday before King's Island and it was working well enough to load up a few pages and emails, but I didn't do a speed test, and since I'm on a limited pool of data, I won't. Any other time I've been on Sprint's network it's been 1x or 3G, which are not pretty in any way, shape, or form. I'll probably go on for another month or so just because it's kind of nice having unlimited minutes for voice calls that can be made over WiFi. I do need a new WiFi router at home as mine is kind of crap but it has worked just fine, even though T-Mobile LTE is solid at my house.
So in closing, coverage overall isn't as good as Verizon, but it is a lot cheaper if you're willing to be conservative with data. You can, at this time, only use the Nexus 6, which I still think is an excellent phone that is now cheaper than ever. I strongly advise those good with T-Mobile or Sprint to give Fi a shot and see how it works for you if you have a Nexus 6 or want to try one on for size. Battery life I have noticed, is considerably better with Fi than with Verizon.. for whatever reason. I'm on 5.1.1 stock but rooted with Franco Kernel, nothing crazy.