The thing that annoys me most is that (as far as I can tell) only the big name providers have integrated mail/caldav/carddav, if you use a bring-your-own-domain solution they need to be added as 3 separate accounts (at least on iOS/macOS).
It has IMAP, CalDAV, CardDAV. It has a snappy and responsive web interface and their iOS app is equally as good.
They have configuration of profiles for MacOS and iOS down (I can't talk to other environments).
I have a bunch of domains doing email through there, but I run my own DNS. I had a need to configure things that Fastmail's DNS doesn't deal with (IPv6 etc) but for general use, it's great. Properly configured SPF, DKIM, DMARC, good spam filtering, easy contact managenment etc.
I've been looking for a sane calendar service that's not Google calendar and there's just nothing that's open-standards compatible.
There's a couple of other good CalDAV servers out there too, though of course as Fastmail CEO I can't really advertise anybody else :p
Will check it out right away :)