While that was my hold up initially, i think i tend to disagree now. I believe the integration is actually pretty on par. For instance if i use siri to create an invent it goes to the stock calendar, but that in-tern goes to many 3rd party calendars automatically.
Likewise if i create an event on 3rd party calendar and then go to my mac, its on the stock calendar now..
The good ones seem to all sync together pretty nicely.
It's still having two calendars that have to sync and managing them etc. It's just an extra step when apple should really just leave the calendar like iOS 6
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different colors for different calendars and a preview pane and there you iOS 6 which had the better calendar app!