TL;DR: Google Fi on an iPhone is iMessage plus Google Wifi calling with awesome international coverage.
Google Project Fi is super interesting as it provides an actual low cost alternative to other carriers especially if you travel. The free data-only SIM is also a nice add-on.
Project Fi is exclusively targeting users of Google Nexus Android devices and you actually need one of the supported phones to activate the SIM which can be ordered on the Fi website.
I currently use an iPhone SE (mainly due to the device's tech specs and form-factor - I can't stand phablets!) so I was curious if I can just buy a Google Fi SIM and use it in an iPhone or any other phone actually. Of course I'm not the first person to think about this, but the only decent article on this topic is this one. Sadly most articles that are returned for a search on iPhone Google Fi are just totally useless. Even this article is not good.
I decided to just order Google Fi and a data-only SIM and give it a try. I used a Nexus 5x that I have access to for activating the SIM card. The activation process is really simple. Basically you need to put the SIM card into a compatible phone and install the Google Fi app. Done.
The activated SIM card can be put into any other phone, I tried an iPhone 5c and it just works. You automatically get the APN settings (the mobile data settings) pushed to your phone. Cellular data immediately works! Voice calls work too.
Wifi calling also works, although it (obviously) only works via the Hangout app but it does work. I put my phone into airplane mode and called the number from another phone and yea it rings.
The only service that is a bit unsatisfying is SMS (text messaging). The default option for Google Fi is to send and receive SMS via Google Hangout. Google Hangout exists for iOS and if you login with your Google Account that is associated with your Google Fi service you just install Hangouts and everything just works! If you actually want to use the iOS Messages app you can deactivate SMS via Hangout in the Hangout app on your phone. This will allow you to send and receive SMS via Messages. The only issue here is that incoming SMS messages get some Google specific data attached, as shown below. This is a little annoying but is only on incoming messages (you don't look like an idiot when sending messages to other people). Most of my contacts are on iMessage anyway these days so this is a non issue. Also I'm ok with using Hangouts for SMS since yea iMessage and other messaging apps.
The switch to change between native SMS and Hangout SMS the switch above it does the same for voice calls (to enable Wifi calling).
The broken* incoming SMS, the ~Dgr... is added by Google Fi, this does not show up in Hangouts. Other people have reported that this just went away after short time.
Things that don't work? switching between T-Mobile, Sprint, and US Cellular since this is done via the Google Fi app on Android devices (I actually don't have any idea about this yet).
Altogether Google Fi looks pretty cool and works with an iPhone (besides the hick-up with SMS). iMessage works (it is just an Internet service after all). Wifi calling via Hangouts is nice.
If you are a hardcore iOS/Mac user Google Fi is too much Google for you. I'm a Linux user with an iPhone so Google Fi makes a lot of sense. Desktop calls and SMS via Hangouts is a nice thing to have in addition to iMessage.
Google Fi on an oooold phone (Android 4.0). Hangouts seem to work fine too.
*The data is a BASE64 encoded blob, no obvious data after looking at a bunch of them of an hour or less.