I purchased my phone originally through google direct, so its an unlocked device, so I thought of doing the fi specific build but never felt like it to restore my settings/apps. I am wondering once marshmallow is released if they will somehow condense the builds into 1 build that will just detect which carrier your on, so their arent so many build versions, but I doubt it.
Lately at home or work I've been doing the airplane mode / wifi only to use less battery and its worked great this way for saving battery. My signal at works bad and the cell radio looking for a signal was horrific on battery life.
The annoying part for me is places I used to have a good sprint signal, with Fi I don't even get a signal or very minimal. Its odd knowing its the same carrier but a toned down service at times.
Overall for the cost I am dealing with it and liking the wifi option for calling. Can't wait for marshmallow.