Holy shitballs do I love Google Fi. Ordered the Nexus 6P on a new Google Fi account. I currently also have a Samsung Note 5 on Sprint.
After hauling both phones around with me all weekend for testing purposes, Google Fi data is faster in every spot that I tested it around my city. Which is odd because Fi supposedly uses Sprint/Tmobile networks, so I assumed both phones would be using the same Sprint network at least some of the time. But it didn't matter, the Google Fi 4G LTE connection was always faster than my Note 5. I thought maybe Fi would be treated like a 2nd rate citizen on other companies networks, getting artificially throttled down a bit or something, but nope. Great speed.
Now as far as the phones go, The Note 5 is still a little superior to the 6P, IMO. They both have the exact same size of screen, yet the Note 5 is overall smaller, while actually showing MORE on the screen because screen real estate isn't being used up by navigation buttons like they are on the 6P. So the Note 5 + physical buttons = more overall usable screen + smaller casing. The front-facing speakers on the 6P are nice, quite a bit louder than the Note 5s bottom speaker, but the quality isn't as good. In listening to the same music files on both, the 6P sounded more treble-heavy, with low/bass notes almost missing from some songs. The Note 5 sounded a lot cleaner and clearer and true on almost all of the music I tested(my own files, Youtube, Spotify)
So all in all, the 6P is still a quality phone, but not quite up to par with top-end phones like the Note 5.