I just bought it today. So far there are only 2 negatives that I can point out.
There is no date on the icon like the default app, so if I want to know the date at a glance in the home screen, I still need the default app to be there.
I don't see an option to invite someone to an event, but whats weird is that if I click on an event that I was previously invited to, I see the option to accept, reject or maybe just like the default one. Other than that, I like the interface better than Apple's. But it still sucks that I have to keep the default app next to the Fantastical one.