I had high hopes for this one, as I'm always a sucker for this sort of fantasy adventure / world-building stuff, and I've wanted to see the show do more with griffons ever since Gilda's initial appearance in Griffon the Brush-Off all the way back in Season One.
Happily, the episode delivers.
The adventure part works nicely, with monsters and perils and Rainbow Dash having another gorge-related accident. The friendship part is equally good, with some quality Pinkie-Gilda moments and an adorable Dash-Gilda flashback. And the worldbuilding part... oh boy, I loved that!
In Greek mythology, the Arimaspi were a race of one-eyed people warring with griffons who lived in the Hyperborean mountains, stealing their gold. Guess what we had here? I love how much the show taps into classic myth and legend, and appreciate this as a nice myth Easter egg to the adult audience, because the primary demographic sure isn't going to notice the connection.