I have recently moved from Gmail. So far, so good. A little more spam coming through compared to Gmail. Pricing is fantastic for small users. IP is not on any blacklists. Delivery looks good even considering all methods the likes of Google deploy today such as various inboxes, etc. Roundcube is not something that I like. There is a bus risk, obviously. :)
I like their pricing structure for bulk accounts. What I'd absolutely love to see is more email services that are Protonmail compatible, utilizing WKD or whatever it they need. Here's something to look at anyway https://github.com/Excision-Mail/Excision-Mail