What's the killer feature that makes this so much more desirable than the calendar baked into iOS? I've pretty much always been happy with it, and Siri integration made it that much better. I don't understand the hatred for the native app.
1. Information density and
2. UX design.
For the first, there is no quick way to see how many appointments you have in a day and you have all sorts of wasted space on a given day view to show you
when something happens using a
spacial method. Fantastical instead uses a rolling list, doing away with chrono-spacial depiction to instead pack more information where it counts.
This leads into the second point: you can scroll endlessly from one day to the next without moving around the app. Pulling down on the calendar provides a day ticker view (packed with chrono-spacial depiction incase you like it) that allows you to quickly jump to a relevant day; your list of appointments follows along smoothly.
These reasons alone make it far superior to the stock calendar app, nevermind their proprietary appointment creation method.