Busycal relies on iCloud for sync. It remains in sync with the Calendar app as well, which means you need to turn notifications off for Calendar. Calendar still comes up when I import an ics file.
One of the nice things is that BusyContacts can maintain a history of calendar items for contacts. It does not have features for sharing holes for scheduling with others, but they may come in the next version.
Of course, versioning means that the “one-time fee” becomes a “rinse and repeat” situation after a year or two.