Their assists animation looks interesting, but I will wait to judge until I can actually try it. The Apple calendar looks great, but feels really clunky to add and edit events. (Especially on a Mac!)
For pure usability, I haven't found a calendar program that beats MS Outlook running on Windows.
Can you elaborate on what you mean by "clunky"? I haven't noticed any issues adding or editing events, in either OS X or iOS's Calendar apps.
Also note that Yosemite added event suggestions when adding new events to the OS X Calendar app (but I don't believe iOS added anything like that for the iOS app). I haven't had a chance to experiment with it yet (since I primarily do event entry in iOS), but it looks pretty straightforward.
For pure usability, I haven't found a calendar program that beats MS Outlook running on Windows.