Currently, for me, lack of a calendar is holding me back. As well as some other features, but I could probably overcome those. Relevant uservoice: https://protonmail.uservoice.com/forums/284483-feedback/sugg...
In general, I should say I am pretty happy with protonmail. I've been using it for over a year now, however, in general I'm still pretty dependent on google for calendar, android, etc.
When it comes to spam, my feeling is that I do get more now. But as long as it is properly categorized -- which it is -- I don't really mind getting it.
> I suppose org-agenda could substitute but the way Inbox organizes all my travel is worth the fact that Google knows i'm flying to London.
Well.. at some point you have to choose something over the other.