This is compelling. I like the "pay as you go" model for data. I'm on an iPhone though and do enjoy visual voicemail - I wonder how much I would miss it? I also wonder what their beta status for iOS means and what their roadmap looks like.
Check compatibility for Apple devices. the one that I notice immediately is that visual voicemail does not work, rather you get the messages texts.
From the Google Fi Compatability checker (https://fi.google.com/compatibility):
"Voicemails won't show in the iOS visual voicemail app, but you'll get them as texts and can call to check them"