Pondering perhaps leaving the big V for Project Fi, but since they use the other guys towers, not sure what sort of coverage they actually have down here. And other places like say China Lake and say Jacksonville, FL. Map looks good, but easy to do that. Looking for firsthand experience.
There are some pros to Project Fi (namely having access to both Sprint and T-Mobile towers at the same time), however if you are already open to using an MVNO and have Sprint or T-Mobile access there are better options, and ProjectFi data charges are in my opinion a bit excessive.
If you like the Project Fi model, using the Sprint network, Republic Wireless/Ting/RingPlus are cheaper and provide more data. Heck, RingPlus regularly has some pretty crazy free offers so I have my old phone (Nexus 5x) on a free plan with 7500minutes, 7500texts, and 7500MB of LTE data. I have no issues tethering, and if I were to go over the extra data is cheaper than Project Fi.
If you like T-Mobile a bit better, you can get the $30 Walmart plan with very minimal minutes/text, but 5GB of LTE data with free speed limited (128k) overage data. And music streaming doesn't count against the data. This pairs very well with a Google Voice number or Ring.to number and using hangouts to make SIP calls using that number.
Or you can use Net10 and get unlimited minutes/texts with 3GB of LTE data (followed by 64k rate limited data) for $36/mo. This works with your choice of T-Mobile, ATT, or Verizon (and is the cheapest Verizon plan around).
Personally, I use Cricket (ATT network, Unlimited/Unlimited/2.5GB) but only because I get crappy T-Mobile reception at my place and don't want to rely on wifi calling or a signal booster.
BTW, if you are considering T-Mobile make sure you get a newer phone that supports LTE band 12 (700mhz) as it will work at much greater distances from the tower and penetrates buildings better. I think all Project Fi phones support band 12.