I actually like PFi a lot. It is great for people who don't use much data and have either good Sprint or T-Mobile coverage in the places they use their phones. The PFi data SIMs are nice as well, it allows you to have a tablet (or other second device) without paying extra other than the data you use on the device.
I should add, if you want to try out Project Fi but aren't sure you want to keep it, so long as you don't have a Google Voice number you are wanting to keep, you can request a new number and then, if you decide you want to keep Project Fi, you can then request that they transfer your number.