I've tried it, but their solution is (IMHO) a very poor fit for a business e-mail account, as there is (or was when I used the service) no way to manage e-mail accounts for your employees and no way to archive / extract their e-mails in case of need (I understand that it's a privacy-focused e-mail service, but as an employer you have legal requirements to keep business documents for several years, and having to rely on your employees goodwill to get the data out of the system is not an acceptable solution for this). Also, there's no calendar integration (https://protonmail.uservoice.com/forums/284483-feedback/sugg...), which again makes it difficult to use this as a business e-mail account.

I've switched to Mailbox.org in 2016 and I'm very happy with their product and service: Their system is based on an open-source solution (OpenExchange) so they don't need to reinvent the wheel and can focus on prodiving good hosting and service (which they do). They also have support for 2FA (including hardware tokens like Yubikeys) and recently revamped their management portal, which allows you to easily create and manage e-mail accounts for your employees.