What separates Fantastical from these others is that its window shows both the free-form entry field and the individual time/date/etc. fields, and as you type in the free-form field, animations show you how Fantastical is interpreting what you’re typing.
This is not just eye candy. The animations are providing instant feedback on how Fantastical is parsing your words and, more important, they’re teaching you Fantastical’s syntax. This is tremendously useful because, despite the wonderful flexibility of NLP, there’s always a syntax and you need to learn it if you’re going to use the product.
It’s a subtle, but extremely important detail of one of the best pieces of software out there today.