I can't speak for Project Fi, but Republic Wireless has a similar model. I mostly run with data off, because free WiFi is abundant, even in rural Vermont.
At home, I have WiFi. At work, I have WiFi. In my car, I have a Freedompop hotspot. Walmart has free WiFi. McDonalds (not that I eat there, but I sometimes pull into their parking lot) has free WiFi. Ace Hardware has free WiFi. Home Depot has free WiFi. My local grocery has free WiFi. The gas station I use most of the time has free WiFi. The main shopping street in Burlington (Church Street) has free WiFi from one end to the other. The mall has free WiFi.
I could go on and on, but effectively, unless I'm driving through the woods (and even then, I have my Freedompop hotspot), practically everywhere I go has free WiFi. The Republic app can automatically accept their terms of service and connect me in the background, if it's enabled (which it is).
I preload audiobooks for long-distance traveling (and just for listening while doing home maintenance and such). My car's head unit has a 64GB flash drive full of music and podcasts. What's the point in paying more than $10 per month? I'm considering Google Fi over RW for only one reason: so I can have calling and texting in Canada, when I'm not near WiFi.