I worked at a company that used Fastmail for ~150 employees. In my personal experience, the email side was rock solid and support was both responsive and helpful. I miss Fastmail's extremely fast web interface and handy features like being able to create smart folders from saved search queries.

Calendaring would have been fine for a smaller company, but it was not suitable at our scale. This is mainly because there's no way to make your free/busy status visible to everyone in the company, so you ended up having to manually share your availability with each person you might want to meet with.

Our sales / marketing / recruiting folks lamented the lack of integration into their external CRM tools, but I'm not in that field so I can't speak to it directly.

Similarly, our IT folks didn't like how it was a separate account system to manage, instead of integrating into some other SSO system or being a source of truth for SSO itself... but again, at a smaller scale that's not a problem.