This sounds really good. The only disappointment is that it seems there is no business model that allows email providers and services like this to provide Unlimited encrypted email (no limitations i.e. Gmail-esque) absolutely free to all users. I'd be willing to gamble that if anyone could sustain this for a couple years, people would leave Gmail in droves, no one I know likes having to use the USA/NSA/google/big bro…
If Google doesn't provide a serious End to End encryption solution for Gmail, then I will probably use it, too, within a year (unless something better comes along, say like a DarkMail-enabled service).
BUT, and it's a big but, I'd only use it for normal e-mails, just because I want to raise a big enough obstacle for NSA to read even my normal e-mails. However, I would not use it for anything too sensitive. I don't trust ProtonMail for that, and since it doesn't have real end to end encryption, you have to trust ProtonMail.