Not the OP, but on my Android device I have an easier time of calendaring since basically any app I choose will show all of the calendars I want (personal, my work calendar, my manager's calendar, my team's calendar) in the same space. For desktop, I did have Sunrise's desktop app until I noticed I wasn't getting a proper sync of some (apparently random) appointments from desktop to cloud/mobile, and even more so because I accepted an appointment via Sunrise and it changed my email alias in the response from my actual address to Sunrise's generic "invitation@email.sunrise.am" so that people then started emailing me at the generic email rather than my actual address.

Most of my calendars are Google Apps. Outlook was my backup for the work calendar using the Google Apps Outlook Sync app (although I've noticed a few sync failures there, too), and just adding additional calendars to Outlook without having all of my email there too looks like it requires some kind of arcane magick. Thunderbird+Lightning got rotated out of the mix quickly due to a lot of issues syncing and getting multiple calendars into it.

Probably because everyone we're communicating with is mixing between Google Apps/webview calendar and Outlook or Office 365, I also find that I regularly can't see an appointment that is emailed to me in my email client as it shows up as an .ics file attachment instead.

So featurewise:

- Most important for me: Easy addition of multiple calendars, where I can just select which calendar a new appt should be added, as easily as I can in my Android clients- I hate using web clients for calendaring, and they never really show multiple calendars well. NOTE: I mean that the calendar has a direct sync to each of the calendars online, not having one main account to which I have to share all of my other calendars. I want one place where I can go to view and create appointments and then have them also show up in their respective accounts. - Supports open standards like CalDAV/CardDAV as well as syncs to Google or Office 365 (I can select a provider like Fastmail that uses CalDAV for my personal calendar, but I'm stuck with Google or O365 for work) - Desktop client, multiple views (agenda, today, week, month) - drag-and-drop of items into the calendar to create a new appointment - "Send to calendar" from an email as an option - Categorization/tagging on an appointment would be nice - Outlook allows this but Google doesn't seem to allow tagging appointments like you can email

A bit of a data dump, but a desktop client that handles multiple calendars well is surprisingly difficult to find in Windows. I happily pay for a number of otherwise free services (Pandora, Evernote, Pocket, Lastpass, etc.), but there really isn't even a pay option.