Project Fi might not work for everybody... if there is no coverage by T-Mobile or Sprint, then you might not want Project Fi. You have to decide what is best for you.
Personally, I live in a large metro area and have great coverage with almost all the carriers... so I really like Project Fi... I have been using it for a couple weeks right now. The best thing is I have a device in which I can use it on any carrier... so if I need to switch to AT&T, Verizon or T-Mobile... personally, I will never choose Sprint ever again, but it works good on Project Fi for me, since it will roll over to T-Mobile if it can not find Sprint.