Switching between networks isn't scary, and in fact it's quite useful.
Usually when you sign up for a phone carrier, you're just getting service from that one carrier. That's not the case with Project Fi, which lets your phone actively switch between both T-Mobile and Sprint. It's a big selling point for the service, and when you add in Wi-Fi calling and texting it looks like quite the network juggling act.
After using Project Fi for some time, it turns out the regular switching between networks isn't all that confusing after all.
Switching between T-Mobile and Sprint
Project Fi, when paired with a capable phone like the Nexus 6, is set up to intelligently choose between T-Mobile and Sprint networks depending on which one is offering a better signal at the time. We don't really know how Google's deals have been worked out or if the phone technically "prefers" one network or the other, and the "Cellular network settings" on the phone are blank except for a single toggle to turn on or off data roaming (and even that's unnecessary, to be honest). That means you don't have any control over which network you choose, but it's not as scary as it may seem.
In our time using the service in the greater Seattle area we've seen the phone latch onto T-Mobile primarily, only moving to Sprint in some rural areas where T-Mobile had weak or no LTE signal to offer. The reliance on T-Mobile here isn't all that surprising considering how good its network is in this area, but of course the situation may be different where you live — you may spend more time on Sprint instead.
The most interesting thing to note is how seamless the transition is between networks when the phone decides to switch. There actually isn't any indication on the phone when you switch, and no matter which network you're on the phone will always display "Fi Network" in the status bar. You can use an app to see which network you're on (we used SignalCheck), and that was the only way we could actually tell that the phone had switched networks. There wasn't any change in the experience that indicated a switch had been made — no delay, no cutoff and no slow data.
The network switching is seamless, and simply expands your speeds and coverage.
While there are technically ways to explicitly choose just one network or the other, we wouldn't actually recommend that. Considering how smooth the transition is between networks, there's little reason to bother with manually switching or sticking with one — and it really defeats the purpose of an automated system like this.
The auto-switching networks has turned out to be absolutely great so far in our testing. We've been able to get great speeds in the denser parts of the city where T-Mobile has historically done better than Sprint, and in a more rural area where our T-Mobile phones didn't have better than EDGE service we actually had Sprint LTE on our Project Fi Nexus 6. The one downside we've found thus far is the lack of simultaneous voice and data on the Sprint network — meaning if you receive a call, you won't be able to use data if the phone happens to be on Sprint at the time.
Of course there are large portions of the country where the T-Mobile and Sprint networks perfectly overlap (or neither have coverage), but in the areas where they don't, you're getting a wider network than you would with a sole carrier in your phone.
Then you add in Wi-Fi
Plain old carrier networks aren't the only story here, there's also a significant part of the Project Fi experience that leans on Wi-Fi (hence the name). The first part of this is built-in Wi-Fi calling and texting, which can utilize your current Wi-Fi connection instead of the cellular networks. This only works when your Wi-Fi connection is deemed fast enough for calls (it doesn't require much speed), but it'll seamlessly drop to the cellular network if necessary without any intervention. This works in the built-in Phone dialer and Google Messenger apps, and doesn't require the use of the Hangouts app (but you can use it if you wish).
This means you can call and text no matter where you are, even out of cellular range, but perhaps the nicest feature about this Wi-Fi calling and texting is that it doesn't require any intervention or settings manipulation on your part. Simply pick up the phone and call or text, and it'll go out over whatever network will handle it. And because your "Fi Basics" charge covers unlimited talk and texts, you don't have to worry about which way it's routed.
The other part of the Wi-Fi story with Project Fi is the so-called "Wi-Fi Assistant" that automatically connects you to open Wi-Fi networks as you move about. This is one of the less-understood parts of the service, but it's pretty basic once you know what to expect. If you choose to keep your Wi-Fi turned on at all times and let the Wi-Fi Assistant also scan for networks (as is the case by default), your phone will automatically connect to any completely open Wi-Fi network that it finds and is "verified as fast and reliable."
It will only connect to networks that are completely open — that is, no password, no splash page, no "click here to connect" or "watch this advertisement to connect" getting in the way. While at first you may think there are plenty of open networks around you, many of them actually require this extra step of a splash page or some kind of authentication that'll keep your Project Fi phone from connecting. Presumably Google is working on some sort of database of these wide-open networks that it can connect to, which should help going forward, but right now it's just scanning as it goes.
It'll connect to fewer networks than you'd think, but when it does connect it works well.
When your phone does find a completely open Wi-Fi network that it can connect to, it'll connect and you'll be using it for data instead of the cellular network. You'll also automatically be connected to a Google VPN service, which is completely free, and tunnels your connection past the inherently insecure network and through Google's servers to hit the outside Internet. This is good for your own security, and also helps give you a consistent data connection. The VPN service isn't available when you manually connect to any network, though — it's only for those that the Wi-Fi Assistant connects to automatically.
Even in an area with lots of available and technically "open" (aka not password-protected) Wi-Fi networks, we never found the Wi-Fi Assistant to latch onto that many networks, mostly due to the limitations noted above. But when it did find a network — such as those in cafes and public places — the connection was solid, and not once have we needed to manually disconnect to use our phone. It never latched onto a network with a bad connection speed, and when we walked out of range of the network it smoothly transitioned back to mobile data.
Living the multiple network life
So long as you don't try to tinker much and simply trust what the phone and service can do automatically, you'll be satisfied with the network switching and automatic Wi-Fi access in Project Fi. Having open access to both T-Mobile and Sprint means there are fewer places where you'll be left without a mobile data connection, and in areas with bad speeds from one of the two carriers there's a good chance the other one can offer some reprieve.
And in places where you have access to Wi-Fi — be it a known network or an open one — the built-in Wi-Fi calling and texting features, along with the help of the Wi-Fi Assistant, will let you keep using your phone in areas with bad mobile networks or just to save you a few dollars a month in data usage.