Hey I didn't know FM were having a go at calendaring. Can you make calendars as slick as you did webmail? I hope so! I tried it out, I think you're on the right track; I could even switch over today, despite some minor features missing (e.g. some views, split TZ start/end). I'll also say, don't be afraid to move beyond the standard paradigms to implement user stories. Calendaring has sucked for years, and reimplementing Google Calendar simply with fewer interface defects would leave a world of needs going begging.
I've used FastMail calendering for several years now, I've got to say I've never had a single issue with it, accepting links / invites from other people on gmail etc... all works perfectly and events sync and update not only quickly, but reliably. I've (been forced to) used O365 for work for years and that shows a stark contrast where events go missing, have incorrect timezones from people in not just the same timezone but same organisation, disconnection issues, many year old appointments popping up at random etc... I've never had any of that with FastMail's calendering - just the way it should be. However I know that this Ask HN is about open-source calendering and FM is a hosted solution. The biggest issue I've personally had with most of the available CalDAV web interfaces I've seen / tried is that they tend to be written in PHP and while I don't want to hate on languages and it's not that simple, PHP isn't something I want to add to my existing web-stack of Ruby and Python which while both far from perfect feel more modern to host / deploy.
